阅读说明：本技术 用于在碱性纺丝浴中湿纺丝纤维素纤维的方法和纺丝路线单元 (Method and spinning route unit for wet spinning cellulose fibers in an alkaline spinning bath ) 是由 本特·哈格斯特龙 托拜厄斯·科恩克 乔纳斯·恩斯特龙 于 2020-02-20 设计创作，主要内容包括：本发明公开了一种用于形成纤维丝束的方法,所述方法涉及湿纺丝程序,所述湿纺丝程序包括以下步骤：将纤维素浆状物溶解于碱性水溶剂中以形成纤维素纺丝原液组合物；在凝固浴中纺丝所述纤维素纺丝原液组合物以产生纤维丝束,所述凝固浴的pH大于7.0,优选地pH至少为10；以及使所产生的纤维丝束经过一系列连续的拉伸和洗涤步骤,其中通过逆流流动洗涤程序用洗涤液体洗涤所形成的纤维丝束。(The invention discloses a method for forming a fiber tow, the method involving a wet spinning procedure comprising the steps of: dissolving cellulose pulp in an alkaline aqueous solvent to form a cellulose dope composition; spinning the cellulosic spinning dope composition in a coagulation bath to produce a fiber tow, the coagulation bath having a pH greater than 7.0, preferably a pH of at least 10; and subjecting the resulting fiber tow to a series of successive drawing and washing steps, wherein the formed fiber tow is washed with a washing liquid by a counter-current flow washing procedure.)

1. A method for forming a fibrous tow, the method involving a wet spinning procedure comprising the steps of:

dissolving cellulose pulp in an alkaline aqueous solvent to form a cellulose dope composition,

spinning the cellulosic spinning dope composition in a coagulation bath to produce a fiber tow, the coagulation bath having a pH greater than 7.0, preferably a pH of at least 10, and

the resulting fiber tow is subjected to a series of successive drawing and washing steps, wherein the fiber tow is washed with a washing liquid of progressively decreasing alkalinity, preferably by a counter-current flow washing procedure.

2. The method of claim 1, wherein the step of dissolving the cellulose pulp is performed in a cold alkaline aqueous solvent at a temperature of 0 ℃ or less.

3. The process according to claim 1 or 2, wherein the process comprises passing the produced fiber tow through at least five successive washing steps, preferably at least seven successive steps, more preferably at least ten successive steps.

4. The method according to any one of claims 1 to 3, wherein at least one washing step is performed by spraying, preferably in at least some of the washing steps with spraying or preferably in each washing step with spraying, a flow rate of spraying washing liquid of at least 5kg washing liquid per kg cellulose spinning dope composition provided to the coagulation bath, more preferably at least 8kg washing liquid per kg cellulose spinning dope composition.

5. The method according to any one of claims 1 to 4, wherein the alkali content is gradually reduced during the washing procedure in the produced fiber tow, calculated on the dry fiber tow, to less than 50ppm by weight NaOH, alkali content being calculated in ppm by weight of NaOH.

6. The method according to any one of claims 1 to 5, wherein the coagulation bath comprises sodium hydroxide and sodium carbonate and/or sodium sulfate, preferably the coagulation bath comprises 3 to 10 wt.% sodium hydroxide, preferably the coagulation bath comprises 10 to 28 wt.% of at least one of sodium carbonate and sodium sulfate.

7. The process according to any one of claims 2 to 6, wherein the cold alkaline aqueous solvent comprises from 0.4 to 1.2 wt.% zinc (calculated as Zn) based on the total weight of the cellulose dope composition.

8. The process according to any one of the preceding claims, wherein the cellulose dope composition comprises cellulose, cellulose carbamate or another cellulose derivative in the range of from 4 to 12 wt. -%, preferably from 5 to 10 wt. -%, based on the total weight of the cellulose dope composition.

9. The method of any one of the preceding claims, wherein the cellulose spin dope comprises urea or cellulose carbamate, which is at least partially hydrolyzed in the coagulation bath and the continuous washing step.

10. The method of claim 8, wherein any ammonia formed by hydrolysis of the cellulose carbamate or urea is collected and discharged from the spinning step.

11. The method according to any one of the preceding claims, wherein the drawing of the fiber tows is performed and controlled by adjusting the speed at which the fiber tows travel from the coagulation bath and through the successive washing steps.

12. The process according to any one of the preceding claims, wherein the speed of the fiber tow of washing step 1 is kept substantially constant or is gradually increased in one or more subsequent washing steps, at least up to and including the washing step after which the fiber tow has a hydroxide concentration below 0.3 wt.%.

13. The method according to any of the preceding claims, wherein the stretching is controlled to such an extent that the total stretching is in the range of 30 to 80%, preferably to such an extent that the tensile strength of the produced fibers becomes at least 15 cN/tex.

14. The process according to any one of the preceding claims, wherein drawing of the fiber tows is performed between the coagulation bath and the first washing step, and wherein the fiber tows are maintained in a drawn state during at least a portion of the process thereafter.

15. The process according to any one of the preceding claims, wherein the drawing of the fiber tows is carried out between the coagulation bath and the first washing step, and wherein drawing is continued during at least two, more preferably at least three, consecutive washing steps, preferably the at least two consecutive washing steps during which drawing is continued are among four washing steps arranged immediately after the coagulation bath, preferably the fraction of the total drawing carried out in the at least two washing steps is at least 25%, more preferably at least 40% of the total drawing.

16. The process according to any one of the preceding claims, wherein a stretched state is maintained during at least 50% of the washing procedure, said stretched state meaning that any significant relaxation in the fiber tows is avoided, preferably the fiber tows are subjected to elongational drawing at least two positions, wherein a first position is between the coagulation bath and the first washing step and a second position is in at least one successive washing step, still more preferably the second position of elongational drawing is within or between the first washing step and the second washing step.

17. The process according to any one of the preceding claims, wherein the fiber tows are subjected to an elongation draw of a certain degree of draw between the coagulation bath and the first washing step, preferably continued during at least three consecutive washing steps, still more preferably continued between and/or within at least three consecutive washing steps of which the total degree of draw is in the range of 0.7 to 1.2 times the draw performed between the coagulation bath and the first washing step, preferably with an additional degree of draw in each step being lower than the additional degree of draw in the steps between the coagulation bath and the first washing step, more preferably wherein the additional degree of draw is reduced in each consecutive step.

18. The method according to any one of the preceding claims, wherein the greatest part of the total elongation draw of the produced fibre tows is performed in a draw step between the coagulation bath and the first washing step of the counter current flow washing program, preferably at least 40% of the total elongation is performed between the coagulation bath and the first washing step of the counter current flow washing program when comparing the elongation of the draw step between the coagulation bath and the first washing step with the elongation of other individual draw steps between and/or within washing steps of the counter current flow washing program.

19. The process according to any one of claims 1 to 17, wherein the maximum portion of the total elongational draw of the produced fibre tows is performed in a drawing step between the first and second washing steps and/or within the first or second washing step.

20. The method according to any one of the preceding claims, wherein the alkaline water solvent comprises zinc, wherein the alkalinity in the fiber tow gradually decreases during the washing procedure, and wherein zinc diffuses out of the fiber tow and into the washing liquid during the washing procedure.

21. A method according to claim 20, wherein zinc diffuses out of the fiber tow and precipitates in the wash liquid in at least one wash step, wherein the wash liquid is suspended to keep the precipitated zinc dispersed in the wash liquid, and wherein the precipitated zinc is passed with the wash liquid without settling to at least one upstream wash step, as seen with respect to the direction of conveyance of the fiber tow.

22. The method of claim 21 wherein the precipitated zinc along the counter current wash liquor is dissolved in the wash liquor of increased alkalinity in at least one upstream wash step and at least partially recycled and reused in the step of dissolving cellulose pulp in an alkaline water solvent to form a cellulose dope composition.

23. The method according to any one of the preceding claims, wherein the washed fiber tow is subjected to a drying operation in which the fiber tow is dried in an unconstrained manner, substantially without severe bending to allow free shrinkage substantially without tension in the fiber direction, to produce a washed and dried fiber tow.

24. The method according to any one of the preceding claims, wherein the method further comprises crimping the fiber tow, preferably after drying the fiber tow.

25. The method of claim 24, wherein mechanically crimping is performed after the drying the fiber tow.

26. The method of claim 24 or 25, wherein the method comprises cutting the dried and crimped fiber tow into staple fibers.

27. The method according to any one of the preceding claims, wherein the fiber tow is treated with acid in an acid treatment step after the last washing step in the washing program.

28. The process according to any one of the preceding claims, wherein one or more surfactants are provided during the process to reduce cohesion between individual filaments in the fiber tow, wherein the one or more surfactants are preferably provided to the fiber tow after the washing procedure, preferably after a possible acid treatment step, and preferably before the drying procedure.

29. A spin path unit for forming fiber tows, the spin path unit comprising:

-a dissolver unit arranged for dissolving cellulose pulp in an alkaline aqueous solvent, preferably in a cold alkaline aqueous solvent, at a temperature of 0 ℃ or lower to form a cellulose spinning dope composition;

-a spinning unit arranged for spinning the cellulosic spinning dope composition in an alkaline aqueous coagulation bath having a pH of more than 7.0, preferably a pH of at least 10, to produce a fiber tow; and

-a washing line, preferably a counter-current flow washing line, comprising several washing units arranged in sequence for washing the produced fibre tows with successively lower alkalinity.

30. The spinning route unit according to claim 29, wherein said washing route comprises at least five washing units, preferably at least seven washing units.

31. The spinning route unit according to claim 29 or 30, wherein said spinning route unit further comprises one or more stretch control units, preferably at least for controlling said elongational stretch between said coagulation bath and said first washing unit, more preferably also for controlling said elongational stretch between and/or within several of said washing steps.

32. The spin route unit according to any one of claims 29 to 31, wherein the spin route unit comprises a fiber tow drying unit adapted to perform a drying operation in which the fiber tow is dried in an unconstrained manner substantially without sharp bends to allow free shrinkage substantially without tension in the fiber direction, preferably the spin unit comprises a crimping unit for crimping the washed fiber tow, still more preferably the spin route unit further comprises a fiber cutting unit for cutting the washed and dried fiber tow into staple fibers, to produce washed and dried fiber tow.

33. The spin route unit of any one of claims 29 to 32, wherein the spin unit further comprises means to enable zinc to diffuse out of the fibre tows and precipitate in a wash liquid and to enable the precipitated zinc to be recycled and reused in the dissolver unit along the wash route, preferably along a counter current wash flow route.

Technical Field

The present invention relates to processes for forming and processing fiber tows, and more particularly to methods of drawing and washing and drying fiber tows. The invention also relates to a spin path unit for forming and processing fiber tows.

Background

Different types of fiber forming processes exist. In the viscose process, derivatized cellulose (cellulose xanthate) is dissolved in NaOH solution and the resulting spinning dope is coagulated in an acidic spinning bath. NaOH from the spin dope and H from the spin bath₂SO₄Are all consumed (chemical reaction), thereby forming Na₂SO₄This chemical is of lower economic value today. Derivatised CS₂A large amount is lost. So that the chemicals cannot be recycled. Another feature of the viscose process is the drawing of the tow after the spinning bath, followed by cutting the fiber tow into staple fibers. After cutting, the staple fibers were spread randomly over a slow moving screen, on which the fibers were washed with water in a counter-current flow. After bleaching and application of the finish, the fiber fleece bed is opened mechanically and the lost fibers are dried with hot air.

In contrast, there are processes using alkaline spinning baths, in which recycling of chemicals is possible. One such method is disclosed in WO2018/169479, which relates to a method for preparing regenerated cellulose fiber compositions. The method comprises providing a dope comprising cellulose and a solution of an additive in an alkaline solvent, wherein the solvent cellulose is present at a concentration of about 5 wt% to 12 wt% and the additive is present in a range of 0.1 wt% to 10 wt%, calculated on cellulose; contacting the cellulose dope with an aqueous coagulation bath fluid having a pH greater than 7, thereby forming a regenerated cellulose fiber composition; and drawing and washing the fiber composition in one or more washing and drawing baths.

A related process is disclosed in EP 3231901 a1, wherein cellulose is dissolved in an aqueous NaOH solution to provide a cellulose dope. The spinning dope is extruded into a coagulating liquid comprising an aqueous solution of sodium salt. EP 3231901 a1 also describes a process for separating (cooling crystallization) and recycling sodium hydroxide and sodium salts from the liquid withdrawn from the coagulation bath and subsequent washing baths.

A preferred process for preparing a dope by direct dissolution of cellulose in cold alkali is described in EP3231899a1, which is suitable for use in combination with the process for forming and processing a fibre tow according to the present invention.

The main difference between the acidic and alkaline spinning baths used to solidify the alkaline cellulose solution is that in the latter case the network of precipitated cellulose filaments becomes highly swollen until the alkali is washed out of the structure, whereas the instantaneous neutralization of the alkali in the acidic spinning bath causes almost instantaneous densification of the fibrillar cellulose structure.

The known process using an alkaline spinning bath thus presents new challenges suitable for the treatment to be carried out, compared to the viscose process using an acidic spinning bath. These challenges relate to, among other things, productivity issues, fiber quality/characteristics issues, and recycling issues.

Disclosure of Invention

The present invention aims to provide an improved process for forming cellulosic fibres, wherein the process is based on a technique involving an alkaline spinning/coagulation bath, and wherein the process is improved in terms of the quality of the fibres produced. More particularly, various aspects of the improved process are described that can be applied alone or in combination to obtain an improvement having one or more of the following advantages:

producing high-strength fibres by drawing a fibre tow under conditions of progressively lower alkalinity and simultaneous washing, so that the orientation of the cellulose molecules obtained is a permanent feature of the fibres and thus prevents the orientation obtained by the drawing process from becoming loose, and/or

Reduce or even avoid damaging the fibre-to-fibre adhesion of downstream operations, such as carding, sliver making and spinning, and/or

To a high degree of washing out zinc compounds from the fibre tow and to facilitate the recycling of zinc for reuse in the preparation of the spinning dope, and/or

Dry wrinkles to reduce deterioration of the fibre strength of the final dried fibre during the drying process, and/or

Obtaining mechanically crimped fibres with retained fibre strength

It has been found that one or more of the above problems relates to the production of fibres in cellulose which is directly dissolved in alkali, preferably cold alkali, and that the regeneration in an alkaline spinning bath can be solved or improved by a process which is characterized in that the fibres leaving the spinning bath are collected in the form of a tow and that these fibres remain in the form of a tow through all the successive processing steps from the spinning bath until finally the fibre tow is cut into short fibres.

One or more of the objects are achieved by a method of forming a fibrous tow, the method involving a wet spinning process comprising the steps of:

dissolving cellulose pulp in an alkaline aqueous solvent to form a cellulose dope composition; spinning the cellulosic dope composition in a coagulation bath to produce a fiber tow, the coagulation bath having a pH greater than 7.0, preferably a pH of at least 10; and subjecting the resulting fiber tow to a series of successive drawing and washing steps in which the fiber tow is washed with a washing liquid of progressively decreasing alkalinity. It should be noted that the stretching may be performed before and/or during the washing step.

With respect to the expression "successive stretching and washing steps", it should be noted that the process according to the invention relates to a process comprising at least two washing steps. However, with regard to stretching, this can be done by different methods according to the invention. An alternative is to draw directly after spinning in a coagulation bath. The fiber tow may then be held in a stretched condition (not necessarily with any set elongation) through the washing steps of the procedure. Therefore, the method and viewing angle will be further explained below.

Furthermore, an alternative to stretching with and without set elongation can be achieved by the washing procedure according to the invention. Furthermore, as a further alternative, the fiber tow may be washed in a slightly relaxed state, wherein the fiber tow is stretched, for example, between the third and fourth washing steps. It should be noted that this is only one alternative and that many other alternatives are fully feasible according to the invention. In addition, alternatives and explanations are given further below.

Preferably, the step of dissolving the cellulose pulp is performed in a cold alkaline aqueous solvent at a temperature of 0 ℃ or less.

Furthermore, preferably, the washing step is carried out by a counter current flow washing procedure.

Further, it is preferable that the washing procedure is performed so as to maintain the tension of the fiber tow until the concentration of sodium hydroxide in the fiber tow is below 0.3 wt%.

As mentioned above, the present invention relates to a process involving spinning a cellulosic dope composition in a coagulation bath having a pH greater than 7.0, preferably a pH of at least 10, and typically also a pH well above 10. This means that the method according to the invention is clearly different from the so-called viscose technology, in which cellulose is passed through CS before being dissolved in alkali₂Derivatization is carried out and the fibers are regenerated in an acidic coagulation bath.

The method according to the invention, in which the alkalinity of the fiber strand is gradually reduced during the drawing and washing process, preferably involves a countercurrent flow washing procedure.

The different viewing angles are discussed further below.

Drawings

Fig. 1A shows a spinning route according to a first embodiment.

Fig. 1B shows a spinning route according to a second embodiment.

Fig. 2 shows a spinning path according to another embodiment.

Fig. 3a and 3b show the drawing of the filament bundle at different basicities.

Figure 4a shows the titer of fibers sampled at different positions along the spinning path.

Fig. 4b shows the composition of the wash liquor of the spinning path referred to in fig. 4 a.

Figure 5 shows the composition of the wash liquor in the elongation tensile test.

Figure 6a is a micrograph of a fiber cross-section of fibers washed in pure water.

Fig. 6b is a micrograph of a fiber cross-section of a fiber gradually washed with successively lower alkalinity.

Fig. 7 shows a test apparatus for testing fiber adhesion.

Fig. 8 is a photograph of fibers washed with progressively lower alkalinity (on the right side of fig. 8 and labeled "B") and fibers washed immediately with plain water (on the left side of fig. 8 and labeled "BW").

Fig. 9a shows on the left side of the photograph: drying the fibers as a tow in an unconstrained manner without sharp bends; the right side of the photograph shows: short fibers dried in a random crimped state.

Fig. 9b shows the strength of the fiber of fig. 9 a.

Figure 10 schematically illustrates mechanical crimping using a stuffer box crimper.

Fig. 11a is a photograph of a dried fiber tow before (right) and after (left) crimping.

Figure 11b shows the strength of the fiber tow before and after mechanical crimping.

Figure 12 shows the washing efficiency of a test in which washing technique immersion and spraying were compared.

Examples and detailed description

In fig. 1A part of a spinning line 1 according to a first embodiment is shown. In this example, the coagulation bath 2 comprises at least three spinning positions or spinning nozzles 3. Each spinning head 3 contains a plurality of spinnerets, and each spinneret includes a plurality of capillaries. The fiber tows from the spinnerets/locations are combined side-by-side into a flat common fiber tow. The fibre tows produced are introduced into a washing program 4, preferably operating according to the countercurrent flow principle. As shown in fig. 1A, there can be up to n washing steps, where n can be at least 5, preferably at least 7, and up to 10 or more. Vn refers to the speed of the tow above each godet 5 in each particular washing step. During the washing procedure, water flows into the final washing step. A washing liquid is then flowed through each washing step counter-currently relative to the fiber tow. The wash liquid (as numbered with reference to the processing of the fiber tow) exiting the first wash step has a higher alkalinity. As indicated in fig. 1A, in each respective washing step, the fiber tows, still under tension, are contacted with the washing liquid in each respective washing step by means of a pump-driven circulation flow of the washing bath liquid, spraying or showering the washing bath liquid onto the fiber tows. Pressure rollers are applied to the exiting tow from each wash step to reduce the amount of wash liquid that is entrained (carried) by the tow from one wash step to the next.

Another embodiment is shown in fig. 1B. As indicated in fig. 1B, in each washing step, the fiber tows, still under tension (meaning that the fiber tows are undergoing elongational drawing, or at least not relaxing), are introduced down into the washing bath, then pulled up from the washing bath and introduced into the next washing bath. A pressure roller is applied to the exiting tow from each wash step to reduce the amount of wash liquid that the tow is entrained from one wash step to the next.

A schematic diagram of a spinning path according to another embodiment is shown in fig. 2. It is noted that in this example, the coagulation bath comprises a spinning head. The fiber tows are produced by introducing the spinning dope into a coagulation bath. The spun fiber tow is then introduced into a counter current washing procedure, which operates as disclosed above. The elongating stretching is performed between some washing steps, or even between each washing step, and also between the coagulation bath and the first washing step. As mentioned above, this elongational stretching may be performed in different ways and with different degrees of stretching in different steps. After the last washing step, a softening step may be provided, in which a surfactant is added to the fiber tow. The tow may be sprayed or subjected to a softening bath or by licking a roll or in some other manner. Thereafter, drying may be performed, followed by crimping and finally cutting of the resulting fibers. The crimping may also be performed before drying or in a semi-dried state (not shown in fig. 2). It should be noted that other steps may also be involved, such as an acid addition step arranged directly after the final washing step and/or the bleaching step.

In the processes shown in fig. 1 and 2, the spin bath liquid or coagulation bath liquid may comprise water, sodium carbonate (Na)₂CO₃) Or sodium sulfate (Na)₂SO₄) Or mixtures thereof, sodium hydroxide (NaOH) and minor amounts of zinc-containing salts. When a fine (e.g., 50 to 70 μm in diameter) dope jet extruded from the spinneret capillary is contacted with the spin bath liquid, water and some hydroxyl ions diffuse out of the jet, while sodium ions and carbonate and/or sulfate ions (assuming sodium sulfate is present in the spin bath) diffuse into the jet due to the difference in osmotic pressure (difference in concentration). Due to the change in chemical composition within the dope jet, the cellulose no longer remains in solution and precipitates in the form of a more or less oriented nanofibrillar network. The degree of orientation of the nanofibrils parallel to the longitudinal direction of the fiber depends on the design of the spinneret capillaries and the draw down ratio employed in the coagulation bath, i.e. the ratio V₀/V_exit。V₀The speed of the filament bundle leaving the coagulation bath, V_exitIs the velocity of the dope jet exiting the spinneret capillary (volumetric flow of dope divided by total capillary cross-sectional area). By the coagulation process, the liquid jets leaving the spinneret capillaries are converted into soft and highly expanded gel filaments which are drawn up through the spinning bath by means of godets and by means of buoyancy. Thus, the coagulation process and the process wherein the spinning bath or coagulation bath comprises sulfuric acid (H)₂SO₄) The viscose process of (a) is very different. In the viscose process, sodium hydroxide in the spinning dope is neutralized with the acid. The cellulose precipitates very rapidly and forms very dense and hard filaments instantaneously at the spinneret exit. This also meansMost of the water in the spinning dope (dope containing about 85% water) ends up in the acid bath and the fiber tow entrains from the spinning bath only about 120% by weight of the spinning bath liquid based on dry cellulose. For the process according to the invention, the dope jet is coagulated into an alkaline spinning bath, the corresponding amount may exceed 1000% by weight of the spinning bath.

In this process, sodium carbonate and/or sodium sulphate is carried along by the filaments. At the same time, some of the water and hydroxyl ions from the spin dope are transferred into the coagulation bath liquid. It has been found that the coagulation bath content can be slowly increased or decreased depending on the amount of spinning bath liquid that is extruded from the tow at the godet and returned to the spinning bath. The force of the pressure roller is preferably adjusted so that the coagulation bath content remains constant or so that the coagulation bath liquid overflows, see fig. 1A. Thus, only salt (Na) is required₂CO₃And/or Na₂SO₄) Addition was continued to the coagulation bath to keep the salt concentration constant. It can be mentioned that this overflow will be even greater if the hydrated form of the sodium salt is added to the coagulation bath to keep the sodium salt ion concentration constant. If not intentionally adjusted by adding NaOH and Zn to the coagulation bath, the concentration of NaOH and Zn in the coagulation bath will be lower.

Maximum elongation is achieved by subjecting the fiber tow to elongation stretching to achieve maximum elongation stretch and thereby maximum fiber tenacity when the fiber tow is in an alkaline state having a relatively high sodium hydroxide content. Such elongational drawing orients the nanofibrils themselves in the longitudinal direction of the fiber. However, if the fiber is not held in a stretched state (such "stretched state" means that the fiber is further elongated, or at least the fiber tow is held at such a tension that it does not significantly relax), then with a further reduction in basicity, the induced orientation will relax to some extent, thereby reducing the fiber tenacity.

To obtain fibers with high tenacity, the tow is elongated and drawn between godets 0 and 1 (the speed of godet 1 is suitably higher than the speed of godet 0, see fig. 2). Elongational drawing is assumed to orient the nanofibrillar cellulose structure in the direction of (along) the fiber tows. It has been found that if the tow is ofThe higher the basicity (higher the NaOH content in the tow), the greater the stretchability of the tow, for example when the tow is between godets 0 and 1 (fig. 2). This is illustrated in fig. 3a and 3b, which show laboratory scale drawing of tows having different basicities. The coagulation bath was maintained at 28 ℃ and contained 20 wt.% sodium carbonate, 5.6 wt.% NaOH and 0.56 wt.% Zn, and the speed of godet 0 (V0) and the speed of godet 1 (V1) and the speed of extrusion from the spinneret (V1)_exit) The same is true. The spinning dope used contained 6 wt% cellulose, 7.5 wt% NaOH and 0.76 wt% Zn and was extruded through a spinneret having 300 capillaries with a diameter of 55 μm. After passing through a bath in which the concentration of NaOH was adjusted to 0%, 2.2%, 3.5% and 4.6% by weight, at godets 1 and 2 (V2)>V1) between the two ends of the fiber tow. Figure 3a shows that the maximum possible draw ratio (V2/V1) increases with increasing weight% NaOH in the bath before elongation drawing, before fiber tow break. However, it has been found that if the drawn tow is cut into staple fibers at this stage (immediately after godet 2), which means that the tension in the fibers is released, the nanofibrils lose orientation to a large extent, resulting in insufficient fiber strength. By gradually reducing the alkalinity of the fiber tow subjected to the maintained tension (meaning that any relaxation of the fiber tow is substantially avoided), it has been found that the fibers remain highly oriented and that this orientation is a permanent feature of the fibers. One possible explanation is that as the basicity gradually decreases in successive washing steps (1-n), the nanofibrils gradually approach each other (decrease in swelling) and bond to each other by forming hydrogen bonds.

To provide one example, tow samples were taken at different locations along the spinning path. The washing unit was designed according to fig. 1A using 10 washing steps (n-10). The number of capillaries in the spinneret was 13500 (capillary diameter 55 μm). The spinning bath was operated at 29 ℃ and contained 18 wt% sodium carbonate, 5.3 wt% sodium hydroxide and 0.5 wt% Zn. The spin dope used contained 6 wt% cellulose, 7.5 wt% NaOH and 0.76 wt% Zn. The ratio of the flow rate of the washing water to the flow rate of the spinning dope was one (Qw/Qd ═ 1), and the temperature of the washing water introduced was 20 ℃. The draw ratio V1/V0 was 1.4. The speed of the godets 2 to 10 is kept constant and equal to the speed of the godet 1, i.e. V1-V2-V3-V4-V5-V6-V7-V8-V9-V10. Tow samples were taken directly after godets 1, 3, 4, 6, 7, 9 and 10 (see fig. 1A and 2) and further free washed in water. After the tow sample was freely dried in air, the titer (dtex) of the individual fibers extracted from the tow sample was measured. Fig. 4a shows the measured titer change, while fig. 4b shows the concentration of NaOH and Na2CO3 in the relevant wash liquor. Fiber titer was theoretically calculated based on the spinning conditions used and assuming a zero relaxation of 1.30 dtex. From the graphs of fig. 4a and 4b it can be concluded that in order to completely avoid orientation relaxation caused by stretching between godet 0 and godet 1 and thus to obtain a final fiber titer of 1.3 dtex, the fiber tow should be kept in a state of being under tension (not relaxed) at least until a washing step in which the washing liquid has a NaOH concentration of about 0.3 wt.%.

With respect to dividing the elongation draw in a counter current washing process, independent speed adjustment of the godets (0-n in fig. 2) can be considered important for several reasons: 1) differential elongation stretching along the wash sequence can be used to optimize fiber properties (tenacity and elongation); 2) to avoid possible slackening of the fibre tows; and 3) to avoid undesirably high tensions in the fibre tow due to shrinkage. Thus, independent speed regulation ensures tight control of the tow tension during (along) the wash.

In one test run, fibers were spun according to fig. 2 using 12 (n-12) washing steps. The elongation stretch was divided as shown in table 1 below.

TABLE 1

The stretching was performed in 4 steps as shown in table 1 to achieve almost the same total stretching. The alkalinity in the wash bath is shown in fig. 5.

As can be seen from table 1 and fig. 5, the final fiber properties can be influenced by the dispersion drawing such that the drawing is performed at different basicities. In this example, it can be seen that the fiber characteristics are improved if the partial elongation drawing is performed at a lower basicity.

In view of avoiding the precipitation of zinc in the fibers, it has been found that zinc can be Zn (OH) if the alkalinity does not gradually decrease during washing₂The form of (a) precipitates inside the fiber. Zinc can be harmful to the aquatic environment and therefore the amount of zinc should be minimized in the final fiber. It is also important that the zinc following the fibres is lost and cannot be recycled in the process, thus incurring additional costs.

In fig. 6a and 6b, SEM micrographs of ion-polished fiber cross-sections of fibers washed in pure water directly after coagulation bath (fig. 6a) and of fibers washed gradually in several washing baths with successively lower alkalinity (fig. 6b) are shown. In fig. 6a, many submicron particles of zinc hydroxide were observed, while the fibers that underwent gradual washing did not show any signs of precipitated particles (see fig. 6 b).

By using a washing principle in which the alkalinity is gradually reduced, Zn follows the NaOH clearly, without precipitating inside the fibers, and diffuses out of the fibers and into the washing liquid. It has been experimentally confirmed that when using a washing principle with gradually decreasing alkalinity, the Zn/Na weight ratio in the leaving washing liquid is the same as the Zn/Na weight ratio in the spinning dope (about 0.1), indicating that Zn and NaOH remain together in the solution and are therefore completely washed out of the fiber. Zn contents of less than 50mg/kg of fibre can then be obtained.

The possible precipitation of Zn inside the fiber thus seems to depend on the concentration gradient over the washing machine/unit. The concentration gradient depends on, among other things, the following factors:

ratio of washing water flow to spinning dope flow

Number of washing steps

The quantity of washing liquid entrained by the tow, which depends on the force exerted on the pressure roller

The above factors are not independent, as the fiber tow needs to be washed to some maximum alkalinity. For example, for the same washing efficiency, if the number of washing steps is reduced, the ratio of washing water to spinning dope must be increased; if the force on the pressure roller is increased (less wash liquid is entrained to the next wash step), the number of wash steps or the ratio of wash water to dope can be reduced.

It can also be a problem when Zn is precipitated in the washing liquor in the form of zinc hydroxide particles. Such particles can lead to precipitation and to possible blockage of connecting lines/pipes between washing steps. Precipitation can be avoided by ensuring sufficient movement of the wash liquid (avoiding stagnation zones/zones) and then the Zn particles will follow the wash liquid to the higher alkalinity wash step, where the Zn is dissolved again. This means that a method is provided which avoids extraction of solid Zn particles from the wash liquor in a downstream washing step for recycling, which method is optional. Furthermore, dilution of chemicals for economic recycling can also be minimized. Since the recycling of NaOH, Zn and sodium salts may involve energy intensive processes such as evaporation of water from the wash liquor, the dilution is related to the process economics. Therefore, the ratio of the flow rate of the washing water to the flow rate of the spinning dope (Qw/Qd) should be minimized. In this respect, counter-current flow washing is a very effective method. Qw/Qd decreases as the number of washing steps (n) increases. Qw/Qd also decreases with reduced entrainment of wash liquid between wash steps, the amount of entrainment in turn being dependent on the squeeze force of the pressure roller (see FIG. 2). The process complexity and investment cost increase with n. During a wash with lower alkalinity, the pressure roller force may be higher downstream. The force of the upstream pressure roll in the washing process, where the alkalinity is high and the fibers are still soft and swollen, must be reduced to avoid damaging the fibers/tows. In fig. 4, an embodiment is given where n is 12 and Qw/Qd is 1. In this example, the force of the pressure roller was lower in the first 3 washing steps and then gradually increased in steps 4 to 12 until the force was about 10 times greater in step 12 than it was in step 1.

Furthermore, in terms of avoiding fiber-to-fiber adhesion during washing away of contained chemicals, it has been found, according to the present invention, that fiber-to-fiber adhesion can be a nuisance if the alkalinity of the tow is suddenly reduced by washing while the tow is under tension (which means not in a relaxed state). By using counter-current flow washing, in which the alkalinity is gradually reduced, the fibre-to-fibre adhesion is eliminated or at least reduced to a minimum.

One test device is shown in fig. 7. According to fig. 7, the tow bale (a bale of about 20 tows) is collected after V1 and washed in wash baths 1-5 with gradually decreasing alkalinity according to table 2 below.

TABLE 2

The sample marked BW in fig. 8 was transferred directly to a washing bath #5 (pure water). The sample labeled B in figure 8 was transferred first to bath #1 and then sequentially to baths # 2-5.

It can be seen that individual fiber tows in the tow bundle are clearly distinguishable in the sample marked BW on the left, while individual tows are difficult to distinguish in the sample marked B on the right, which indicates that fiber-to-fiber adhesion can be largely avoided by gradual washing off of chemicals from the tows.

The method according to the invention also provides a method of avoiding low strength fibres due to dry wrinkles by drying fibre tows rather than short fibres. Due to the higher crystallinity, regenerating the fibers in directly dissolved cellulose results in fibers that are rigid and somewhat brittle (higher dry and wet modulus than conventional viscose fibers). High fiber stiffness is considered a positive factor for the dimensional stability of the garment during washing. However, such fibers are susceptible to dry wrinkles, creating stress concentrations when unfolded and stretched. Weak points of staple fibers during carding/sliver formation/spinning can result in lower tenacity, shortened fibers, and dust generation.

It has now been found that stress concentrations which reduce the strength of the fibres due to dry wrinkles can be avoided if the fibres are dried in the form of a tow before being cut into staple fibres.

Several repeated experiments conducted have shown that drying randomly crimped fibers as in conventional viscose processes results in a reduction in fiber strength compared to unconstrained drying (i.e., at zero tension) of a fiber tow that is substantially free of sharp bends. Fig. 9a shows on the left side: drying the fibers as a fiber tow in an unconstrained manner and substantially free of sharp bends; shown on the right side: short fibers dried in a random crimped state. In fig. 9b, the tenacity (strength in cN/tex) of fibers dried in an unconstrained manner, substantially without sharp bends, referred to in fig. 9b as "free-dried straight tows", is shown compared to staple fibers dried in a random crimped state. As shown, fibers dried in an unconstrained manner to a fiber tow and substantially free of sharp bends have consistently higher strength than fibers dried to staple fibers in a random crimped state.

Further, a method for mechanical crimping without causing a reduction in fiber strength is provided. As described above, drying out wrinkles can be avoided by drying the tow. However, due to poor cohesion within the web, straight fibers without crimp are difficult to handle in downstream operations (such as carding and sliver making) because straight fibers are less likely to entangle with each other than bent/crimped fibers. It has now been found that mechanically crimping the dry tow prior to cutting into staple fibers using, for example, a stuffer box crimper (see schematic drawing of crimping principle in fig. 10) can be a remedy.

It has been found that fiber tows dried in an unconstrained state (i.e., not under tension during drying); the tow is allowed to shrink freely in the machine direction/fiber direction during drying, resulting in crimped fibers with retained strength.

Several repeated experiments performed showed that the fiber tow dried in an unconstrained manner and substantially free of sharp bends (referred to as "free-dried tow" in fig. 11 b) retained mechanical properties after crimping, see fig. 11a and 11b to compare the tenacity of the uncrimped and crimped tows. In fig. 11a, the right sample is the dry fiber tow before crimping and the left sample is the dry fiber tow mechanically crimped by a stuffer box such as the one shown in fig. 10. In fig. 11b, the strength (tenacity in cN/tex, bars on the left in fig. 11 b) of the uncrimped samples of fiber tow is compared to the strength (bars on the right in fig. 11 b) of the samples of fiber tow that have been mechanically crimped. As can be seen from fig. 11b, the tenacity before and after mechanical crimping is substantially the same. The tests described with reference to fig. 9a and 9b clearly show that the "drying knots" formed during drying create weak spots on the fibres, whereas the tests of fig. 11a and 11b clearly show that the rather severe folds or bends formed in the crimper by the already dried fibres (10% to 20% moisture) do not create any such weak spots.

With respect to crimping using a stuffer box crimper, it has been found that the dry content of the tow entering the stuffer box cannot be too high, nor too low. If the dry content of the tow approaches 100 wt.% (very low humidity; as measured by gravimetric measurement by weighing the wet sample before drying and then weighing the sample after drying in an oven at least 100 ℃ (such as about 105 ℃) for at least 1 hour (such as 2 hours or more, even up to 24 hours), assuming weight loss as evaporated water, the fibers can become very rigid and brittle to break in a stuffer box. If the moisture content of the tow is too high (low dry content), the fibers become so soft that the stuffer box becomes clogged. Best results and best smooth processability occur if the dry content of the tow entering the filling box is in the range of 80 to 90 wt.%.

In addition, it was also in the test in which immersion and spraying were comparedThe washing efficiency was investigated. Washing efficiency W_EThe following calculation can be made:

this corresponds to the NaOH concentration (or Na) between the input and output tows₂CO₃Concentration) was divided by the NaOH concentration (or Na) between the input tow and the wash liquor₂CO₃Concentration). In this context, "input tow" means tow entering the washing step and "output tow" means tow exiting the washing step.

As indicated in fig. 12, it is noteworthy that spraying provided enhanced washing efficiency compared to immersion in the two different test devices. In these tests, the total fiber tow had 243,000 dtex (corresponding to 162,000 filaments with a titer of 1.5 dtex). The width of the fiber tow is limited to 5cm, resulting in a theoretical tow thickness of 48,600 dtex/cm.

The total wash time was 20 seconds for all samples shown in fig. 12. This time is related to the time the tow is immersed in the wash liquid in the immersion box and the time the tow is exposed to the spray stream in the spray box, respectively. Furthermore, in the spray test, at a dope flow rate of 328kg/h (═ 19.7kg/h dry fiber (cellulose)), the washing liquid flow rate was 10,600kg/h, corresponding to 10,600kg/h/328kg/h ═ 32.3kg washing liquid flow rate per kg of dope composition. It should be noted that the given flow rate may be very different from the flow rates used in these tests, e.g. lower, but also higher. Furthermore, in the immersion test 650 litres of washing liquid were used, wherein the volume of washing liquid was recirculated at the same flow rate rating as in the spray test. In the tests, relatively high liquid flow levels have been used. Thus, according to the invention, it is entirely possible to use lower grades. In fact, any type of traffic class may be used in accordance with the present invention.

It is noted that in the comparative tests the washing efficiency of the spray washing was kept above 80%, for example up to about 95%, which should be compared to immersion, which is 28% and 51.1% respectively. Based on these results, according to one embodiment of the present invention, at least one washing step is performed by spraying, preferably all washing steps are performed by spraying.

In view of the above, the methods and systems disclosed herein provide various preferred methods for addressing several fiber quality/characteristics and recycling issues. Some embodiments of such preferred methods are summarized below.

1. Maximum draw is obtained by drawing the fiber/tow in the alkaline state (high concentration of NaOH in the tow) to obtain maximum fiber tenacity. However, if the fiber is not kept in a stretched state (meaning relaxation is substantially avoided), the induced orientation will relax to some extent in case the alkalinity is further reduced (NaOH and sodium salts are washed away).

2. The mechanical properties of the fibers can be positively influenced if the total elongational draw is divided into a plurality of steps at successively lower basicities. The independent speed adjustment of the godets 1 to n can also be used to avoid possible slackening of the tow or to reduce the undesirably high tensions in the tow due to shrinkage when chemicals are washed out of the tow.

3. To improve the economics of chemical recycling, dilution of the chemicals with water after the spinning bath should be kept to a minimum.

4. When the bulking fibers are still in close proximity, the rapid decrease in alkalinity in the bulking fibers will result in undesirable fiber-to-fiber adhesion, thereby rendering it difficult to separate the fibers in downstream operations. Therefore, such a rapid decrease in alkalinity is preferably avoided.

5. Because of the higher crystallinity, setting the fibers in directly dissolved cellulose results in fibers that are stiff and somewhat brittle (both dry and wet modulus higher than conventional viscose fibers). High fiber stiffness is considered a positive factor for the dimensional stability of the garment during washing. However, it has been found that such fibers are susceptible to dry wrinkling, creating stress concentrations upon unfolding and stretching. Such wrinkles are easily formed if the fibers are dried in a random crimped state, as would be the case if conventional viscose fiber technology (washing and drying of cut staple fibers) were to be applied. Weak points of staple fibers during carding/sliver formation/spinning can result in lower tenacity, shortened fibers, and dust generation. By drying the fibers in the form of fiber tows prior to cutting and drying the fibers in an unconstrained manner, such dry wrinkles can be avoided, thereby improving fiber strength.

6. Drying out wrinkles can be reduced or even avoided by drying straight fibres. However, straight fibers without crimp are difficult to handle (poor web cohesion) in downstream operations such as carding and sliver making. Preferably, the mechanical crimping is performed without causing a reduction in fiber strength, and preferably, such crimping is performed after at least partially drying the fiber tow and before cutting the fiber tow into staple fibers.

Some or even all of the above listed problems associated with producing fibers in cellulose dissolved in alkali and coagulation in alkaline coagulation baths may be solved by using one or more of the embodiments described herein.

Embodiments of the invention

Embodiments of the present invention are discussed in more detail below.

According to one embodiment, the method comprises passing the produced fiber tow through at least five consecutive washing steps, preferably at least seven consecutive steps, more preferably at least ten consecutive steps. According to one embodiment, the number of washing steps is in the range of 10 to 20. When multiple steps are used, less wash water is required. This is advantageous in terms of recovery economy, since less water must be treated.

According to the present disclosure, each washing step may be considered a single unit operation. The fiber tow produced by spinning from the cellulose dope composition in the coagulation bath is subjected to a subsequent washing step. Each individual washing step may be considered as an operation in which the fiber tows are treated by entering and passing through the respective washing step and then exiting the washing step. Each washing step may, for example, involve passing the fiber tow through a wash bath, i.e., the tow is immersed in a wash liquid, or the tow may be sprayed with a wash liquid, or a combination of immersion and spraying may be employed. According to one embodiment of the invention, at least one washing step is carried out by spraying, preferably in at least some of the washing steps with spraying or preferably in each washing step with spraying, a flow rate of spraying washing liquid of at least 5kg washing liquid per kg cellulose spinning dope composition provided to the coagulation bath, more preferably at least 8kg washing liquid per kg cellulose spinning dope composition. Spraying is discussed further below with respect to comparative testing of immersion washes (see fig. 12).

However, it should be noted that the individual washing steps need not be limited to a particular washing bath. It should also be noted that the term "bath" merely means that the fiber tows are in contact with the washing liquid of the washing step under consideration. Such contacting may be performed in a variety of ways and does not necessarily refer to a literal "bath". The washing step is defined as washing the tow with a washing liquid having a composition different from the composition of the washing liquid in the upstream and/or downstream washing steps. Such separation of the washing liquid composition between the washing steps can be achieved, for example, by pressing the incoming fibre tows so as to reduce the residual of washing liquid from the preceding washing step, and the fibre tows in a particular step are subjected to the particular washing liquid of the washing step for a certain residence time and are, for example, mechanically and/or hydraulically processed on one or several rolls into a very thin and flat shape to cause convection/displacement of the liquid within the fibre tows and then pressed out of the washing liquid again when processed out of the washing step. In this connection, it may be mentioned that the more washing liquid that is pressed out of the fibre tow when moving the fibre tow from one washing step to a subsequent washing step, the more efficient the washing procedure, which means that fewer washing steps are required and/or less washing water has to be used.

The wash liquor in each successive washing step can be characterized by its chemical composition concentrations in terms of sodium hydroxide (NaOH) and a coagulating salt (e.g., Na2CO3 or Na2SO4 or mixtures thereof). The tow entering the washing step has a higher NaOH and salt concentration than the wash liquor in the washing step. The concentration of NaOH and salt in the input tow is conveniently defined based on the liquid fraction of the tow (excluding cellulose). The tow exiting the washing step has a lower NaOH and salt concentration than the tow entering this step (again, excluding cellulose), but the NaOH and salt concentration in the exiting tow is still typically somewhat higher than the wash liquor, except in the case where the tow is in equilibrium with the wash liquor in terms of interdiffusion of chemicals. The relative washing efficiency of the washing step under consideration can be described in terms of the NaOH and salt concentrations in the exiting tow relative to the corresponding concentrations in the entering tow and the washing liquid in contact with the tow in the washing step under consideration.

Obviously, the washing efficiency depends on a number of factors that can be influenced by the design of the washing unit and the applied processing conditions. Examples of such factors are the temperature and residence time of the fiber tow in contact with the washing liquid, mechanically and/or hydraulically induced convection of the washing liquid within the tow, and the thickness of the fiber tow, among others.

According to one embodiment, the alkali content is gradually reduced during the washing procedure in the produced fiber tow, calculated on the basis of dry fiber tow, to less than 50ppm by weight NaOH, the alkali content being calculated in ppm by weight of NaOH.

According to another embodiment, the coagulation bath comprises sodium hydroxide and sodium carbonate or sodium sulphate, preferably said coagulation bath comprises 3 to 10 wt.% sodium hydroxide, preferably said coagulation bath comprises 10 to 28 wt.% sodium carbonate or sodium sulphate or a mixture thereof.

As described in WO2015/000820, the coagulation bath suitably comprises sodium hydroxide and sodium carbonate as it relates to the recycling of alkali in the cellulose spinning process. Accordingly, other additives are also possible in the process of the invention. In addition to the sodium hydroxide added to the coagulation bath by the spinning dope, the coagulation liquid is composed in such a way that it is a poor solvent for the cellulose, whereby new fibers are formed during the release of the sodium hydroxide into the bulk of the coagulation bath.

Sodium hydroxide is also suitably recovered. Furthermore, according to one embodiment, after concentration and optional purification, the sodium hydroxide recovered from the counter current washing step is at least partially recycled for use in preparing a new spinning dope.

The concentration of NaOH and salt in the coagulation bath is determined by the rate and composition of the entering spin dope, the rate of salt addition and the entrainment of the coagulation bath liquid by the fiber tows exiting the coagulation bath and the overflow of the coagulation bath liquid (to the recycle stream), if any, which in turn involves the entrainment of the coagulation bath liquid by the fiber tows, which depends on the extrusion force applied to the tows exiting the coagulation bath. The maximum solubility of the salt in the coagulation bath is determined by the temperature and the NaOH concentration.

The salt is important for driving the salting-out process. Since the salt concentration in the coagulation bath liquid is higher than the salt concentration in the cellulose dope composition, water is drawn from the dope jet exiting the capillaries of the spinneret. At the same time, carbonate and/or sulfate ions enter the dope jet (filament). This also means that cellulose molecules bind to each other to form crystalline nanofibrils (cellulose precipitate).

According to one embodiment, the alkaline water solvent comprises from 0.4 to 1.2 wt.% zinc (calculated as Zn), more preferably from 0.6 to 0.9 wt.%, zinc, based on the total weight of the cellulose dope composition. It will be appreciated that other percentages would be obtained by calculating the zinc, rather than, for example, zinc oxide. Since the spinning dope contains Zn, Zn will also be present in the coagulation bath.

According to one embodiment, the cellulose dope composition comprises cellulose, cellulose carbamate or another cellulose derivative in a range of from 4 to 12 wt. -%, preferably from 5 to 10 wt. -%, based on the total weight of the cellulose dope composition. According to one embodiment, the cellulose dope composition comprises cellulose in the range of 5 to 8 wt.%, or comprises cellulose carbamate or a mixture thereof in the range of 5 to 10 wt.%. However, it should be noted that other cellulose derivatives, including but not limited to cellulose ethers and cellulose esters, may also be present in the dope. Additives may be present in the spin dope, such additives may be, for example, zinc compounds and/or various forms of urea. In embodiments where the dope comprises cellulose carbamate or urea, the carbamate or urea will at least partially hydrolyze in the alkaline coagulation bath and subsequent alkaline fiber washing steps. Thus, according to one embodiment, the cellulose spin dope comprises urea or cellulose carbamate, which is at least partially hydrolysed in the coagulation bath and in the successive washing steps. Any ammonia formed by hydrolysis of the cellulose carbamate or urea can be collected and discharged from the spinning step.

Further, as described above, according to one aspect, the method involves a stretching procedure. According to one embodiment, the drawing of the fiber tows is performed and controlled by adjusting the speed at which the fiber tows travel from the coagulation bath and through the successive washing steps.

According to one embodiment, the speed of the fiber tow of washing step 1 is kept substantially constant or is gradually increased in one or more subsequent washing steps, at least up to and including a washing step after which the hydroxide concentration of the fiber tow is below 0.3 wt.%. It should be noted in this connection that the expression "substantially" should be understood such that the method may involve that the speed may be reduced or at least not gradually increased in a shorter time.

According to the invention, the stretching is carried out during a countercurrent washing procedure, i.e. between or within different washing steps. Furthermore, it should be noted that the drawing is also carried out between the coagulation bath and the first washing step. Alternatives and embodiments regarding where more or less stretching is performed are discussed further below. Furthermore, a preferred aim of the drawing in the process according to the invention is to ensure that the fiber tows are drawn in at least a first part of the washing procedure, i.e. between the coagulation bath and the first washing step, and between and/or within the first washing step and the second washing step. This is because too low a draw in these steps will affect the quality of the fiber tow more than too low a draw in the later steps. It should be noted, however, that the method according to the invention also covers alternatives for stretching along most or even the entire washing program, i.e. between all washing steps, and also covers programs, etc., in which the tension is reduced at an early stage for a short period of time.

As disclosed above, in each washing step in the washing program according to the invention, the alkalinity is reduced. By gradually reducing the alkalinity of the fiber tow subjected to the maintained tension, it has been found that the fibers obtained by drawing the fiber tow maintain a high degree of orientation (meaning that the fibers are oriented in the longitudinal direction of the fibers) and that this orientation is a permanent feature of the fibers. With regard to stretching and alkaline environment, it may also be mentioned that the total stretching during the washing operation may be divided into several stretching steps with successively decreasing alkalinity.

Furthermore, according to yet another embodiment of the present invention, the stretching is controlled to such an extent that the total elongation is in the range of 30 to 80%, preferably to such an extent that the tensile strength of the produced fiber becomes at least 15 cN/tex. The expression "controlling the stretching to such an extent that the total elongation is in the range of 30% to 80%" means that the fiber tow has been stretched to an elongated state so that the length is increased by 30% to 80% when compared with the initial length of the fiber tow (i.e., before the start of stretching).

According to one embodiment of the invention, the drawing of the fiber tows is performed between a coagulation bath and a first washing step, as illustrated above, and wherein the fiber tows are kept in a drawn state during at least a part of the process thereafter. It should be noted that the fiber tow may be maintained in a stretched state during one or several subsequent washing steps.

According to another embodiment of the invention, the drawing of the fiber tows is performed between the coagulation bath and the first washing step, and wherein the drawing is continued during at least two, more preferably at least three consecutive washing steps, preferably said at least two consecutive washing steps during which the drawing is continued are among four washing steps arranged immediately after the coagulation bath, preferably the part of the total drawing performed in the at least two washing steps is at least 25%, more preferably at least 40% of the total drawing. The percentages given above refer to the share with respect to the total stretch, i.e. are given as a percentage of the total elongation.

According to another embodiment, the fiber tow is drawn such that the drawn state is maintained during at least 50% of the washing procedure, preferably during at least the first 50% of the washing procedure, preferably at least between the coagulation bath and the first washing step and in at least one successive washing step the fibers are drawn to the drawn state. For the above, the expression "stretched state" means a state in which the fiber tow is stretched to a stretched state, or at least is kept under a tension sufficient to keep the fiber tow at its current length, so as to avoid any significant slack. Based on the above description, it should be noted that the expression "maintaining the stretched state during at least 50% of the washing procedure" may also be understood such that "any significant relaxation of the fiber tows is avoided during at least 50% of the washing procedure", i.e. with respect to the "elongated state" of the fiber tows means: avoiding slackening of the fibre tows and/or stretching the fibre tows in order to obtain elongation, the latter also resulting in avoiding slackening of the fibre tows. According to the above, according to one embodiment, the drawn state, which means avoiding any significant relaxation in the fiber tows, is maintained during at least 50% of the washing procedure, preferably the fiber tows are subjected to elongational drawing at least two positions, wherein the first position is between the coagulation bath and the first washing step and the second position is in at least one successive washing step, still more preferably the second position of the elongational drawing is within the first washing step or between the first washing step and the second washing step.

Furthermore, as mentioned above, according to the above-described embodiments, such a state is maintained during at least 50% of the washing program, preferably during at least the first 50% of the washing program. In this context, a percentage relates to a share of the total number of washing steps. Furthermore, as noted, the fiber tows are preferably drawn to an elongated state in an early step of the washing procedure, such as between the coagulation bath and the first washing bath or step, and between and/or within, for example, the first washing step and the second washing step. When sodium hydroxide (NaOH) is washed from the fiber tow, the cellulose nanofibrils bind to each other, preventing the stretched filaments from relaxing or retracting. It is therefore important that the fiber tow does not relax in at least early steps of the washing procedure, such as in the steps from the first washing step to the second washing step and between the second washing step and the third washing step, preferably up to the washing step where the NaOH concentration is still higher than 0.3 wt.%.

According to one embodiment of the invention, the fiber tow is subjected to an elongation stretching of a certain elongation degree between the coagulation bath and the first washing step, wherein the stretching is continued during at least three successive washing steps, preferably wherein the additional elongation degree in each step is lower than the additional elongation degree in the step between the coagulation bath and the first washing step, more preferably wherein the additional elongation degree in each successive step becomes smaller. This embodiment means that the elongation stretching is performed between the coagulation bath and the first washing step (bath), preferably in this first step between the coagulation bath and the first washing step, the stretching is performed with the highest fraction of the total elongation stretching. According to another embodiment of the invention, the largest part of the total elongation draw of the produced fiber tows is performed in the drawing step between the coagulation bath and the first washing step of the washing program, preferably at least 40% of the total elongation is performed between the coagulation bath and the first washing step of the washing program, when comparing the elongation of the drawing step between the coagulation bath and the first washing step with the elongation of the other individual drawing steps between and/or within the washing steps of the washing program.

It should be noted that in the process according to the invention, the elongation stretching, which is the greatest fraction of the total elongation stretching, does not have to be carried out between the coagulation bath and the first washing step (bath). For example, according to one embodiment of the invention, the elongation of the largest portion is performed between the first washing step and the second washing step. Thus, according to one embodiment of the invention, the largest part of the total elongational draw of the produced fibre tows is performed in the draw step between the first and second washing steps and/or within the first or second washing step. Also in this embodiment, the drawing of the relatively large fraction of the total elongation draw is preferably performed between the coagulation bath and the first washing step, although the drawing of the largest fraction is performed in consecutive steps, i.e. between the first washing step and the second washing step.

According to yet another embodiment of the invention, the fiber tows are subjected to an elongation stretching of a certain degree between the coagulation bath and the first washing step, preferably the elongation stretching is continued during at least three consecutive washing steps, still more preferably the elongation stretching is continued between and/or within those at least three consecutive washing steps having a total elongation degree in the range of 0.7 to 1.2 times the elongation performed between the coagulation bath and the first washing step. With regard to the above, it should be noted that the expression "in the range of 0.7 to 1.2 times" refers to the total elongation in three consecutive steps when added together, rather than the elongation in each of the steps thereof.

Furthermore, it should be noted that all elongations may be performed between the coagulation bath and the first washing step together with within or between the first washing step or the second washing step, e.g. all elongations may be performed between the coagulation bath and the first washing step together with within the first washing step or between the first washing step and the second washing step.

Along the washing path, and as shown in fig. 1, rolls or so-called godets are arranged. The speed of the godet or roller can be controlled to control the elongation draw or to avoid slackening of the tow, as the case may be. This may also be the case when controlling the elongation stretching in the process according to the invention. Thus, according to one embodiment, the elongating draw of the tow in each respective washing step and/or avoiding slackening of the tow is controlled by controlling the speed at which the fiber tow is drawn past the respective godet roll of that washing step. Therefore, the independent speed adjustment of the godets 1 to n is also important for avoiding possible slackening of the fiber tow or for reducing the undesirably high tensions in the tow due to shrinkage when chemicals are washed out of the tow.

There are other possible techniques to control the elongation draw and/or to avoid relaxation of the fiber tows, and these techniques may also be used in accordance with the present invention.

Furthermore, as indicated above, one aspect of the process relates to the optimal treatment of zinc in alkaline coagulation bath spinning, preferably using a subsequent counter current wash. Hereby, according to one embodiment, the alkalinity in the fibre tow gradually decreases during the washing program, and wherein zinc diffuses out of the fibre tow and into the washing liquid during the washing program, preferably a counter-current flow washing program. However, it is of interest to prevent the alkalinity (concentration of sodium hydroxide) from suddenly decreasing, as this may result in zinc precipitating within the fibers that make up the fiber tow.

According to one embodiment, zinc diffuses out of the fiber tow and precipitates in the washing liquid in the form of Zn-containing particles in at least one downstream washing step, wherein the washing liquid is suspended to keep the precipitated zinc dispersed in the washing liquid, and wherein the precipitated zinc is dispersed with the washing liquid without settling to at least one upstream washing step. An upstream washing step means a lower number of washing steps since it is the opposite direction of the moving fiber tow. In general, it is in the 4 th to 6 th washing steps that zinc precipitation in the washing liquid is observed. Furthermore, it may also be mentioned that the suspension of the washing liquid to keep the precipitated zinc dispersed in the washing liquid may be achieved by different methods, such as by using a circulation pump or an agitator.

As the NaOH concentration in the wash liquor approaches about 2 wt%, zinc generally begins to precipitate (as zinc hydroxide). The fiber tow carries the zinc particles to the subsequent step. At the same time, the zinc particles are transported in the opposite direction to the washing liquid. This means that the zinc particles are visible in several washing steps (i.e. upstream and downstream) in the vicinity of the washing step with a NaOH concentration of about 2 wt.%. Accordingly, in accordance with the present invention, the alkalinity is gradually reduced enough to allow the zinc to be washed from the fibers without precipitating as zinc-containing particles inside the fibers. Furthermore, at least the three (3) first washing steps preferably have an alkalinity or sodium hydroxide concentration of at least 2 wt.%. Thus, the zinc does not crystallize inside the fibers, while some portion of the zinc leaving the fiber tow precipitates in the liquid of the subsequent washing step. Thus, the wash liquid comprising precipitated zinc particles may be transferred to an upstream wash step with higher basicity, in which such solid zinc particles are again dissolved in solution, i.e. at higher basicity levels. It should be noted that the above-mentioned NaOH concentration (about 2 wt%) depends on the zinc concentration in the washing liquid when zinc hydroxide starts to precipitate. Of course, zinc has some solubility even at NaOH concentrations of about 2 wt.%. Precipitation will occur when the zinc concentration is above the solubility limit at a particular NaOH concentration. For a dope prepared to contain about 7.5 wt% NaOH and about 0.76 wt% Zn, based on the total weight of the dope, a limiting concentration of about 2 wt% NaOH is effective. If the concentration of Zn in the dope is above about 0.76 wt%, it is expected that precipitation will begin at a concentration of NaOH slightly above about 2 wt%. If the concentration of Zn in the dope is below about 0.76 wt%, it is expected that precipitation will begin at a NaOH concentration slightly below about 2 wt%.

According to yet another embodiment, the precipitated zinc along the counter-current wash stream is dissolved in the wash liquor of increased alkalinity in at least one upstream wash step ("upstream" being relative to the direction of movement of the fiber tow, i.e. in the direction of the lower wash step number) and at least partially recycled and reused in the step of dissolving the cellulose pulp in the cold aqueous alkaline solvent to form the cellulose dope composition. This means that there is a recycling of zinc, which is achieved by effectively using the washing route, i.e. without additional recycling loops or the like.

Further, according to yet another aspect, the method further comprises drying the fiber tow. According to one embodiment, the washed and otherwise treated fiber tow in wet condition is subjected to a drying operation in which the fiber tow is dried in an unconstrained manner, substantially without severe bending to allow free shrinkage without any tension in the fiber direction, to produce a washed and dried fiber tow. The expression "substantially free of sharp bends" means that the wet fiber tow is dry when uncrimped. It should be noted that the fiber tows can be bent into a smooth curve as long as no sharp angles are arranged along the fiber tows, wherein preferably the radius of any bend is larger than 10 mm.

Further, according to another embodiment, the method further comprises crimping the fiber tow. The curling can be performed before or after drying. Furthermore, according to another embodiment, the mechanical crimping is performed after drying the fiber tow. By crimping the dry fiber tow according to the present invention, crimped high strength fibers can be obtained. Many different types of crimpers may be used, such as mechanical crimpers, for example stuffer box crimpers. It should be noted, however, that the process may instead include crimping of the semi-wet fiber tow, i.e., the fibers are crimped prior to completely drying the fibers.

According to another embodiment, the method comprises cutting the dried and optionally crimped fiber tow into staple fibers. Thus, according to this embodiment, the tow is first dried and then cut, optionally crimped as well before being cut into staple fibers.

There are additional steps that may be part of the method. According to one embodiment, the fiber tow is treated with an acid in an acid treatment step after the last washing step in the washing procedure. Different acids may be used, one example being a weak acid such as carbonic acid in water. In addition, it is contemplated that the pH may vary. The acid treatment step is performed as a neutralization, which may also provide stronger fibers. In addition, after this step, a further washing step may be included.

According to another embodiment, the fiber tow is treated with a bleaching agent in a treatment step after the last washing step in the washing program.

According to another embodiment, one or more surfactants are provided during the process to reduce the cohesion between the individual filaments in the fiber tow, wherein the one or more surfactants are preferably provided to the fiber tow after a washing procedure, preferably after a possible acid treatment step and a bleaching step, and preferably before a drying procedure. One or more surfactants are typically provided in a so-called softening bath. The surfactant may also be provided by spraying or sprinkling the fibrous tow or by using a lick roll.

The invention also relates to a spin path unit for forming a fiber tow, the spin path unit comprising:

a dissolver unit (e.g. according to EP3231899a1) arranged for dissolving cellulose pulp in an alkaline aqueous solvent, preferably in a cold alkaline aqueous solvent, at a temperature of 0 ℃ or lower to form a cellulose spinning dope composition;

-a spinning unit arranged for spinning the cellulosic spinning dope composition in an alkaline aqueous coagulation bath to produce a fibre tow, the alkaline aqueous coagulation bath having a pH of greater than 7.0; and

a washing line, preferably a counter-current flow washing line, comprising several washing units arranged in sequence for washing the produced fiber tows with successively lower alkalinity. As indicated above, the washing route preferably operates according to the countercurrent flow washing principle.

It should also be noted that all embodiments and alternatives mentioned above in connection with the method according to the invention are also possible embodiments in connection with the spinning route unit according to the invention. This means that when the unit is arranged for performing these steps, different steps can be rewritten.

Some embodiments involving a spinning route unit are shown below. According to one such embodiment, the wash route comprises at least five wash units, preferably at least seven wash units. According to yet another embodiment, the spinning route unit further comprises one or more draw control units, preferably at least for controlling the elongational draw of the fibre tows between the coagulation bath and the first washing unit, more preferably also for controlling the elongational draw of the fibre tows between and/or within several of the washing steps.

Furthermore, according to another embodiment, the spin path unit comprises a fiber tow drying unit adapted to perform a drying operation in which the fiber tow is dried in an unconstrained manner, substantially without sharp bends, to allow free shrinkage without any tension in the fiber direction, to produce a washed and dried fiber tow, preferably the spin path unit comprises a crimping unit for crimping the washed fiber tow, still more preferably the spin path unit further comprises a fiber cutting unit for cutting the washed and dried fiber tow into short fibers. Furthermore, the spin path unit may further comprise means enabling zinc to diffuse out of the fiber tow and precipitate in the washing liquid, and enabling the precipitated zinc to be recycled and reused in the dissolver unit along the washing path, preferably along the counter-current flow washing path, in the upstream direction ("upstream" is relative to the direction of travel of the fiber tow).