Field of the invention

The present invention relates to a process for dissolving cellulose, to provide a cellulose dope for extrusion into an aqueous coagulation bath liquid. The process comprises an adaptation step affecting the solubility of the cellulose to be dissolved.

Background

Fibers and films have large application areas in the textile and packaging industries. For example, cellulose fibers have for long been used in textile industry for making fabric. Typically, the cellulose fibers have been obtained from cotton. There is however a growing interest also in other sources of cellulose, such as wood.

An interesting alternative for obtaining regenerated cellulose fibers is the regeneration of cellulose fibers from solutions of dissolved non-derivatized cellulose, e.g. dissolving pulp. Such a process is described inter alia in EP3443151B1 and WO2018169479A1. By using non-derivatized cellulose, use of e.g. CS2 (carbon disulphide) which is used in the viscose process, may be avoided. To some extent non- derivatized cellulose is soluble in cold aqueous sodium hydroxide and aqueous sodium hydroxide has been used as cellulose solvent in experimental procedures, though no industrial process being economically feasible is available so far.

In order to improve the solubility of cellulose, it may be depolymerized by alkaline or acidic depolymerization reducing the degree of polymerization. Further, the depolymerization treatment as such may also physically affect the cellulose and separates the polymer chains, thereby facilitating dissolution. This kind of treatment is typically denoted adaptation or ageing.

In the art, alkaline as well as acidic processes are known. Both have their pros and cons. If using acidic adaptation, the cellulose has to be washed and neutralized before being dissolved. Furthermore, removal of low molecular weight components of the pulp is limited. Acidic adaptation is a however a robust procedure. The acidity of the pulp during the adaptation treatment, as well as the temperatures used in various stages of the process, may be varied quite a lot, still providing a high quality cellulose dope.

In alkaline adaptation, the neutralization step may be omitted, and in addition removal of low-molecular weight components is efficient. However, in the case of alkaline adaptation, the resulting quality of cellulose dope is very sensitive to the alkalinity of the pulp during the adaptation treatment, as well as the temperatures used in various stages of the process. Furthermore, the final degree of polymerization of the adapted pulp is highly sensitive to the amount of transition metals either present in the original pulp, leaching from the processing equipment, or being added as a catalyst.

In EP 3 292244 a method of treating cellulose pulp for use in regeneration of cellulose is disclosed. The method comprises heating an alkaline slurry comprising cellulose and a metal hydroxide solution having a concentration of 5 to 8% by weight to a temperature of 60 to 80°C, and dissolving the treated cellulose pulp in an alkaline solution having a temperature within the range of -10°C to 12°C. According to EP 3 292 244, the alkaline slurry is to be heated for up to 24 hours, such as for 2-8 hours. Further, an accelerator, such as Mn ²⁺ , typically has to be added. Still the residence time in the heating step is long even if performed under an overpressure (5 to 15 bar) of oxygen. Using accelerators may be troublesome, as it may result in discoloration of the cellulose dope. Furthermore, a fairly high concentration of NaOH has to be used in order to keep the reaction time reasonably short.

Thus, there is a need in the art for an efficient process for providing a cellulose dope.

Summary of the invention

Consequently, according to a first aspect there is provided a process for providing an alkaline cellulose spin dope for extrusion into fiber spinning, film forming, or three-dimensional structures. Such extrusion is well-known in the art. Examples of three-dimensional structures comprises cellulosic sponges, and moisture sorption aerogels. In order to improve the solubility of cellulose, it is typically treated in a process known as adaptation, or ageing, before being dissolved.

In the present process, cellulose is firstly dispersed in a first aqueous sodium hydroxide solution, to form a dispersion of cellulose. This first solution typically comprises 4 to 10 % by weight of sodium hydroxide, such as 5 to 8 % by weight of sodium hydroxide. The cellulose to be dispersed is preferably cellulose pulp, such as dissolving pulp, paper grade pulp, and/or pulp from recycled resources, such as paper, cardboard or textile waste. According to an embodiment, the cellulose is non- derivatized cellulose. Further, also low derivatized cellulose may be used. Such low derivatized cellulose may have a degree of substitution (DS) of not more than 0.1.

While even a DS of not more than 0.1 is fairly low, it is preferred to employ low derivatized cellulose with an even lower DS, such as a DS of not more than 0.05. Thus, the low derivatized cellulose may be provided by derivatization processes employing ethylene oxide or propylene oxide. Derivatized cellulose is preferably not derivatized by a process employing carbon disulfide (CS2). The degree of substitution (DS) for low derivatized cellulose is according to an embodiment less than that degree of substitution (DS) that would be required to be able to completely dissolve more than 1 g of substituted cellulose in 50 g of an aqueous sodium hydroxide solution comprising 8 wt% sodium hydroxide, without additional additives, at +10°C.

Once dispersed, the dispersion of cellulose is heated to adapt the cellulose for its subsequent use in a cellulose spin dope by partly degrading it in a heating step. In order to efficiently and rapidly adapt the cellulose, the dispersion of cellulose is heated to a temperature of more than 100°C. Preferably, the dispersion of cellulose is heated to a temperature of 101 to 125°C, such as to a temperature of 105 to 115°C. In order to provide rapid adaptation, the residence time in the heating step may be within the range of 5 to 90 minutes, such as 7.5 to 60 minutes, or 10 to less than 30 minutes. The residence time is not to be too long, as this may result in a too low degree of polymerization (DP), i.e. a too high degree of degradation, due to the depolymerization of the cellulose. Further, a too short residence time may result in insufficient adaptation, the resulting cellulose being more difficult to dissolve. In the step of heat-treating the cellulose, the concentration of cellulose may be up to 50 % by weight, such as 15 to 50 % by weight.

The adaptation mainly results from heating the cellulose in the presence of alkali. While being performed at high temperature, it may still be of interest to further affect that adaptation, i.e. increase the degradation rate; especially in the absence of addition of accelerators, such as metallic salts of Mn ²⁺ , Co ²⁺ , Fe ²⁺ , Fe ³⁺ and/or Cu ²⁺ , or in the case of only minor addition of such salts. Given the high catalytic activity of such metallic salts at high temperatures, they are rather causing difficulties to optimize and control the degradation and can be eliminated from the process.

It was found that also at a temperature above the 100°C, the degradation rate is fairly low at atmospheric pressure, i.e. an oxygen partial pressure of about 0.21 bar. According to an embodiment, the dispersion of cellulose is hence heated under an over pressure of oxygen, as oxygen affects the adaptation by increasing the degradation rate. The dispersion of cellulose may thus be heated at an oxygen partial pressure exceeding the atmospheric partial pressure of oxygen. The dispersion of cellulose may be heated at an oxygen partial pressure of 0.25 to 10 bar, such as 0.5 to 5 bar, or 1 to 4 bar. Contrary to case with accelerators in the form of metallic salts, the adaption may be controlled and optimized by adjusting the partial pressure of oxygen. Further, also the temperature and the reaction time may be adjusted to control and optimize the adaption.

Once heat-treated, the heat-treated cellulose is to be dissolved in a second aqueous sodium hydroxide solution provide an alkaline cellulose spin dope. The process thus comprises a step of dissolving the heat-treated cellulose in a second aqueous sodium hydroxide solution. In order to dissolve the heat-treated cellulose, the second aqueous sodium hydroxide solution comprises 5 to 12 % by weight of sodium hydroxide, such as 6 to 10 % by weight of sodium hydroxide. Further, in order to dissolve cellulose, the temperature has to be low. The temperature of the second aqueous sodium hydroxide solution in dissolving the heat-treated cellulose is thus +12°C or lower. Typically, the temperature of the second aqueous sodium hydroxide solution in dissolving the heat-treated cellulose is -20°C to +10°C, -10°C to +5°C, or - 5°C to +5°C.

Given that the residence time is to be fairly short in the heat-treatment step, it may be preferred to quench the degradation once the treatment has been completed in order to abort the adaptation or least significantly reduce the degradation rate. Thus, the process may further comprise a step of quenching the degradation of the cellulose. This can be done by releasing, at least partly, an over pressure of oxygen, by cooling the heat treated cellulose, or a combination thereof.

According to a preferred embodiment, the dispersion of cellulose is heated under an over pressure of oxygen. Once completed, the adaptation, i.e. degradation of the cellulose, is quenched by, at least partly, releasing the over pressure, such as releasing the pressure to remove the over pressure. According to an embodiment, the over pressure is decreased to atmospheric pressure within 5 minutes, such as within 2 minutes or even within 1 minute.

Alternatively, the adaptation may be quenched by cooling down the heat- treated cellulose before dissolving it in the second aqueous sodium hydroxide solution. In cooling the heat-treated cellulose down, it may at least be cooled down to temperature of 50°C. In order to quench the adaptation, or least significantly reduce the degradation rate, and improve the control over the adaption, the cooling is typically rapid. Thus, the temperature of the heat-treated cellulose may be decreased from the adaption temperature, i.e. a temperature of more than 100°C, such as 101 to 125°C, or 105 to 115°C, to 50°C or less in less than 10 minutes, such as in less than 7 minutes, or even in less than 5 minute.

The heat-treated cellulose may be cooled down in different ways. It may be cooled down indirectly or directly. The heat-treated cellulose may be cooled down indirectly by subjecting it to a surface being cooled. As example, the vessel in which the heat treatment is performed may be provided with a heating/cooling jacket. This may be used to firstly heat the cellulose by circulating a heated liquid in the heating/cooling jacket. Subsequently, a cooling liquid may be circulated in the heating/cooling jacket to cool the cellulose, whereby quenching the degradation.

In embodiments according to which the dispersion of cellulose is heated under an over pressure of oxygen, the vessel in which the heat treatment is performed may be of various kinds. The vessel should allow for heating the cellulose and subjecting it to an over pressure of oxygen. Further, it should preferably allow for rapidly decreasing the pressure to quench the reaction. The heating may for example be performed in a helix dryer, a batch mixer, or a kneader reactor.

The heat-treated cellulose may also be cooled down directly by adding and mixing a cooling liquid into the heated dispersion of cellulose. The cooling liquid may have a temperature of 25°C or less. The cooling liquid may be water. Further, the cooling liquid may be a fourth aqueous sodium hydroxide solution.

In order to remove impurities, such as low molecular weight cellulosic components and/or non-cellulosic components from the pulp, the process may comprise a step of washing the heat-treated cellulose before dissolving it. The washing step will thus provide a narrower molecular weight distribution. The heat-treated cellulose may be washed with a washing liquid, such as water. Further, the heat-treated cellulose may be washed with a fourth aqueous sodium hydroxide solution before being dissolved in the second aqueous sodium hydroxide solution. The washing step may be combined with the step of cooling the heat-treated cellulose. Thus, the washing may also serve to lower the temperature of the heat-treated cellulose slurry, such as to a temperature of 50°C or lower. The washing liquid may have a temperature of 25°C or less.

According to an embodiment, the dispersed cellulose is steeped in the first aqueous sodium hydroxide solution before being heated. The steeping may serve to distribute sodium hydroxide evenly into the pulp to provide for a desired, narrow molecular weight distribution and favorable dissolution performance in the heating step. Further, steeping may facilitate removal of unwanted low molecular weight components. The dispersed cellulose may be steeped at a moderate temperature below 100°C, such as at a temperate of 10 to 70°C, such as 10 to 30°C. The residence time in the steeping step may be 5 to 120 minutes, such as 15 to 60 minutes. Before heating the steeped cellulose, it may be dewatered to concentrate it. The concentration of cellulose in the dispersion may be 0.5 to 15 % by weight, such as 2 to 12 % by weight, before the dewatering. The concentration of cellulose in the dispersion may be 15 to 50 % by weight after the dewatering. By dewatering the dispersion, the volume to be heated is decreased, whereby less energy is required to heat the dispersion.

According to an embodiment, dispersed, steeped cellulose is diluted before being heated. After dilution, the dispersed, steeped cellulose to be heated may comprise 3 to 9 % by weight of sodium hydroxide, such as 4 to 8 % by weight of sodium hydroxide.

According to an embodiment, the dispersed cellulose is steeped in the first aqueous sodium hydroxide solution before being heated. Further, the steeped cellulose is dewatered to concentrate it. In dewatering the steeped cellulose, a concentrated dispersion of cellulose is provided. Further, a third aqueous sodium hydroxide solution depleted of cellulose is provided. The third aqueous sodium hydroxide solution may however comprise hemicellulose having been dissolved in the steeping step. Thus, the dewatering, apart from decreasing the volume, which is to be heated, may also serve to remove components, e.g. hemicellulose, which may impair the quality of the extruded objects (i.e. fibers, films, or three-dimensional structures). Furthermore, removed components, e.g. hemicellulose, has a value of their own and may thus be of interest to take advantage of. Thus, the third aqueous sodium hydroxide solution may be filtered to remove hemicellulose, whereby providing a filtrate. The resulting filtrate is depleted of hemicellulose and comprises sodium hydroxide. This filtrate may be re-cycled. It may thus be used to form part of the first aqueous sodium hydroxide solution. Further, or alternatively, it may be used to form part of the second aqueous sodium hydroxide solution. The hemicellulose filtered off may be concentrated and purified and valorized. As an example, it may be upgraded to furfural, levulinic acid, xylitol and/related chemicals.

By steeping and dewatering the dispersed cellulose, the molecular weight distribution of the heat treated cellulose may be affected, as a low molecular weight fraction may be removed already prior to the heating. Further, the steeping may at least partly depolymerize the low molecular weight fraction, facilitating removal thereof. The low molecular weight fraction may be quite substantial and the initial molecular weight distribution may even be bi-modal. Steeping and dewatering may thus serve to provide cellulose with narrower molecular weight distribution being unimodal.

As the steeping may serve to remove hemicellulose, or at least reduce the amount of hemicellulose, the pulp used to prepare the alkaline cellulose spin dope does not necessarily have to be a dissolving pulp as commonly has been the case in art. According to an embodiment, cellulose to be dispersed is paper grade pulp or pulp from recycled resources, such as paper, cardboard and/or textile waste.

Further, the dispersion of cellulose to be heated may further comprise other oxidants comprising oxygen, such as hydrogen peroxide and ozone.

Given that the adaptation is performed at relatively high temperature, it may not be necessary to further speed up the adaptation significantly. Any acceleration should be carefully handled in order to avoid significant effects on the cellulose. As an example, the partial pressure of oxygen should not be too high. As already mentioned, it should preferably not exceed 5 bar; more preferably not exceed 2.5 bar.

Further, metallic salts of Mn ²⁺ , Co ²⁺ , Fe ²⁺ , Fe ³⁺ and/or Cu ²⁺ may be added to be present in heating the dispersion of cellulose. Addition of such salts may serve to further affect that adaptation. As already mentioned, given that the adaptation is performed at relatively high temperature, it may not be necessary to further affect that adaptation. Further, any acceleration should be careful in order to avoid a too drastic effect on the cellulose. Thus, it may according to some embodiments be of interest to not add any metallic salts of Mn ²⁺ , Co ²⁺ , Fe ²⁺ , Fe ³⁺ and/or Cu ²⁺ or to only add a minor amount. According to an embodiment:

- the concentration of Mn ²⁺ in the dispersion of cellulose to be heated is lower than 50 ppm, such as lower than 25 ppm; and/or

- the concentration of Co ²⁺ in the dispersion of cellulose to be heated is lower than 1 ppm, such as lower than 0.5 ppm; and/or

- the concentration of Fe ²⁺ in the dispersion of cellulose to be heated is lower than 50 ppm, such as lower than 25 ppm; and/or

- the concentration of Fe ³⁺ in the dispersion of cellulose to be heated is lower than 50 ppm, such as lower than 25 ppm; and/or

- the concentration of Cu ²⁺ in the dispersion of cellulose to be heated is lower than 25 ppm, such as lower than 10 ppm.

The adaptation serves to provide a cellulose suitable for being dissolved in alkali and subsequently extruded into fiber spinning, film forming, or three-dimensional structures. Important properties of cellulose are inter alia the molecular weight, as well as the molecular weight distribution. According to an embodiment:

- the heat treated cellulose to be dissolved in the second aqueous sodium hydroxide solution has a DP of 140 to 600, such as 180 to 600, 200 to 400, 160 to 400, or 180 to 300 (Intrinsic viscosity may be converted to DP and vice-versa with formulas outlined for example in ASTM D4243 - 16 “ Standard Test Method for Measurement of Average Viscometric Degree of Polymerization of New and Aged Electrical Papers and Boards", and/or

- an intrinsic viscosity according to IS05351:2010(E) of 115 to 450, such as 150 to 450ml/g, 190 to 300ml/g, 130 to 300 ml/g, or 140 to 230 ml/g; and/or

- the molecular weight distribution of the heat treated cellulose to be dissolved in the second aqueous sodium hydroxide solution is unimodal.

It has been shown that alkaline provides lower polydispersity index Mw/Mn compared to acidic adaptation.

Before the cellulose is to be extruded and regenerated, it needs to be dissolved. As already outlined, the heat-treated cellulose is dissolved in the second aqueous sodium hydroxide solution. This solution may comprise 5 to 12 % by weight of sodium hydroxide, such as 6 to 10 % by weight of sodium hydroxide. The cellulose is dissolved at a temperature of +12°C or lower, such as -20°C to +10°C, -10°C to +5°C, or -5°C to +5°C, to provide the alkaline cellulose spin dope. In order to promote dissolution of cellulose, the second aqueous sodium hydroxide solution comprises zinc oxide (ZnO). Typically, the second aqueous sodium hydroxide solution comprises 2.7 % by weight or less of zinc oxide (ZnO), such as 0.1 to 2.7 % by weight, or 0.5 to 1.6 % by weight, of zinc oxide (ZnO).

The alkaline cellulose spin dope may comprise 4 to 12 % by weight, preferably 5 to 8 % by weight cellulose.

Further advantageous features of the invention are elaborated in embodiments disclosed herein. In addition, advantageous features of the invention are defined in the claims.

According to a second aspect, being related to the first aspect, there is provided a process for providing an alkaline cellulose spin dope for extrusion into fiber spinning, film forming, or three-dimensional structures.

The process according to the second aspect comprises the steps of:

- dispersing cellulose in a first aqueous sodium hydroxide solution comprising

4 to 10 % by weight of sodium hydroxide, such as 5 to 8 % by weight of sodium hydroxide, to form a dispersion of cellulose;

- heating the dispersion of cellulose to a temperature of at least 80°C, e.g. more than 100°C, such as 101 to 125°C, or 105 to 115°C, under an over pressure of oxygen to partly degrade the cellulose, in a heating step; preferably the residence time in the heating step being 5 to 90 minutes, such as 7.5 to 60 minutes, or 10 to less than 30 minutes;

- quenching the degradation of the cellulose by, at least partly, releasing the over pressure; preferably the over pressure is decreased to atmospheric pressure, within

5 minutes, such as within 2 minutes, or even 1 minute; and

- dissolving the heat-treated cellulose in a second aqueous sodium hydroxide solution comprising 5 to 12 % by weight of sodium hydroxide, such as 6 to 10 % by weight of sodium hydroxide, the second aqueous sodium hydroxide solution having a temperature of +12°C or lower, such as -20°C to +10°C, -10°C to +5°C, or -5°C to +5°C, to provide an alkaline cellulose spin dope. Aspects disclosed herein above in relation to the first aspect are mutatis mutandis applicable also to the second aspect. Brief description of the drawings

These and other aspects, features and advantages of which the invention is capable of will be apparent and elucidated from the following description of embodiments of the present invention, reference being made to the accompanying drawing, in which:

Fig. 1 depicts a flow chart;

Fig. 2 shows the how the viscosity decreases over time according to an embodiment;

Fig. 3 shows the how the viscosity decreases over time according to an embodiment; and

Fig. 4 shows the how the viscosity decreases over time according to an embodiment.

Detailed description

According to the schematic flow chart in Fig. 1, cellulose pulp 20 is dispersed 71 in a first aqueous sodium hydroxide solution 10, typically comprising about 5 to 8 % by weight of sodium hydroxide, to provide a dispersion of cellulose pulp comprising about 2 to 12 % by weight cellulose. Once dispersed, the cellulose pulp is steeped 72 in the first aqueous sodium hydroxide solution 10 for some time, such as about 5 to 120 minutes at 10 to 70°C, e.g. 30 minutes at 60°C. Subsequently, the steeped cellulose pulp is dewatered 73 in a e.g. filter press to provide a concentrated dispersion of cellulose pulp comprising about 15 to 50 % by weight of cellulose (e.g. about 30% by weight cellulose) and a third aqueous sodium hydroxide solution 12 comprising hemicellulose, respectively.

The third aqueous sodium hydroxide solution 12 is filtered 78 to remove hydrolyzed hemicellulose 30. The resulting filtrate, which is depleted of hemicellulose and comprises sodium hydroxide, is routed back to the dispersion 71 of cellulose pulp 20 to supplement the first aqueous sodium hydroxide solution 10.

The concentrated dispersion of cellulose pulp is pressurized by oxygen, such as to about 4 bar, and heated 74 to a temperature above 100°C, to adapt the cellulose by partly degrading it. Once the adaptation is completed, the degradation is quenched 75 by releasing the over pressure. The heat-treated cellulose pulp is washed 76 with a fourth aqueous sodium hydroxide solution 13. Subsequently, the heat-treated cellulose pulp is dissolved 77 in a cold (e.g. -5°C), second aqueous sodium hydroxide solution 11 comprising about 8% by weight of sodium hydroxide to provide an alkaline cellulose spin dope 50 comprising about 5 to 10 % by weight of cellulose. Before being used, the alkaline cellulose spin dope may be filtered. Without further elaboration, it is believed that one skilled in the art may, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative and not limitative of the disclosure in any way whatsoever.

Although the present invention has been described above with reference to specific embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the invention is limited only by the accompanying claims and other embodiments than the specific embodiments described above are equally possible within the scope of these appended claims.

In the claims, the term "comprises/comprising" does not exclude the presence of other elements or steps. Additionally, although individual features may be included in different claims, these may possibly advantageously be combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous.

In addition, singular references do not exclude a plurality. The terms "a", "an", “first”, “second” etc. do not preclude a plurality.

Examples

The following examples are mere examples and should by no means be interpreted to limit the scope of the invention, as the invention is limited only by the accompanying claims.

Example 1

Dissolving grade cellulose pulp (intrinsic viscosity of about 450 ml/g) was steeped in 8 wt.% NaOH solution under constant mixing for 30 min at 60°C. The consistency of the steeping slurry was 2.5 wt.%. After the steeping, the dispersion was dewatered and its consistency adjusted to 30 wt.%. The dewatered pulp (i.e. a concentrated dispersion of cellulose) was shredded into a homogeneous mass for 10 minutes in a pulper, which had been pre-heated to 60 °C. The shredded pulp was transferred to four Teflon-lined autoclaves, which had been pre-heated to 110 °C. The autoclaves were pressurized with 4 bar oxygen, and mounted in a 110°C oil bath and rotated to ensure uniform heating of the pulp. After treatment for 8 minutes, the intrinsic viscosity of the pulps were measured at 4-minute intervals (cf. Fig. 2).

Adaptation for 20 minutes, provided a pulp in the form of a heat-treated cellulose with intrinsic viscosity of 186 ml/g, i.e. just below the target value 200 ml/g, as can be seen in Fig. 2 (cf. dashed line). After washing with water, the heat treated cellulose was dissolved in a sodium hydroxide solution (around 10 wt.% sodium hydroxide) comprising about 1 wt.% ZnO) at temperature of -4 °C, to provide alkaline cellulose spin dope in the form of a solution comprising 6 wt.% cellulose and having a clogging value K _r = 477.

Example 2

Dissolving grade cellulose pulp was steeped and adapted in the same manner as in Example 1, except for that the autoclaves were not pressurized. After treatment for 20 minutes, the intrinsic viscosity of the pulps was measured at 20-minute intervals (cf.

Fig. 3)

As can been seen from Fig. 3, the degradation rate was very slow when the autoclaves were not pressurized by oxygen catalyzing the degradation. Thus, it was concluded that the degradation may be quenched by releasing the over pressure.

Example 3

Dissolving grade cellulose pulp (intrinsic viscosity of about 450 ml/g) was steeped in 4.5 wt.% NaOH solution under constant mixing for 30 min at 60°C. The consistency of the steeping slurry was 2.5 wt.%. After the steeping, the dispersion was dewatered and its consistency adjusted to 30 wt.%. The dewatered pulp (i.e. a concentrated dispersion of cellulose) was shredded into a homogeneous mass for 10 minutes in a pulper, which had been pre-heated to 60 °C. The shredded pulp was transferred to four Teflon-lined autoclaves, which had been pre-heated to 110 °C. The autoclaves were pressurized with 4 bar oxygen, and mounted in a 110°C oil bath and rotated to ensure uniform heating of the pulp. After treatment for 25 minutes, the intrinsic viscosity of the pulps were measured at 5-minute intervals (cf. Fig. 4).

Adaptation for 30 minutes provided a pulp in the form of a heat-treated cellulose with intrinsic viscosity of 189 ml/g, i.e. just below the target value 200 ml/g, as can be seen in Fig. 4 (cf. dashed line). After washing with water, the heat treated cellulose was dissolved in a sodium hydroxide solution (around 10 wt.% sodium hydroxide) comprising about 1 wt.% ZnO at a temperature of -4 °C, to provide alkaline cellulose spin dope in the form of a solution comprising 6 wt.% cellulose and having a clogging value K _r = 111.

Thus, the degradation rate could be adjusted by lowering the NaOH concentration. Still, efficient degradation could be accomplished in short time.

Example 4 (comparative example)

Dissolving grade cellulose pulp was steeped and adapted in the same manner as in Example 1, except for that the treatment temperature was lowered from 110°C to 60°C and that the cellulose concentration was 12 % instead of 30 %. After treatment for 100 minutes (intrinsic viscosity about 360 ml/g), the intrinsic viscosity of the pulps was measured at 50-minute intervals. Even after treatment for 250 min, the intrinsic viscosity still was about 240 ml/g, i.e. well above 200 ml/g. Thus, it seems that also a metallic catalyst (e.g. Mn ²⁺ ), as taught by EP 3 292244, has to be added in order to accelerate the degradation to operate effectively at lower temperatures.