Coagulation of cellulose solution in citric acid

Abstract

According to an example aspect of the present invention, there is provided a method for manufacturing cellulose films and / or coatings from alkaline cellulose solutions via citric acid regeneration coagulation.

Description

COAGULATION OF CELLULOSE SOLUTION IN CITRIC ACIDFIELD

[0001] The present invention relates to regeneration of alkaline cellulose solutions in citric acid into films and coated paper and carton boards.BACKGROUND

[0002] Alkaline cellulose solutions are typically coagulated in a sulphuric acid bath that regenerates the dissolved cellulose rapidly and prevents efficient orientation of cellulose molecules. As a consequence, the mechanical properties of for example wet-spun fibres are compromised.

[0003] Patent documents US 2016 / 0369456 Al and US 2018 / 215893 Al disclose methods and systems for processing cellulose-containing materials and for isolating cellulose molecules for use in a variety of downstream applications. In particular applications, the disclosure relates to methods and systems for treatment of cellulose- containing materials to isolate cellulose molecules and to produce regenerated polymers, fibers, and / or fabrics from the isolated cellulose molecules.

[0004] As another example of existing prior art solutions, Tu et al. (2020) and CN 110042488 disclose a method for spinning robust cellulose filaments that were produced from a cellulose solution in a NaOH / urea / ZnO aqueous system with a relatively high concentration (7.5 wt %) via the wet-spinning technique by coagulating in cheap and renewable 15 wt % citric acid / 5 wt % trisodium citrate / 40 wt % glycol at low temperature, followed by drawing in a 5% sulfuric acid bath.

[0005] Similarly, it is typical in the present technical field to use sulphuric acid coagulation baths in the process of coating paper and carton boards.

[0006] There is however a need for a replacement of sulphuric acid based regeneration coagulation methods for cellulose, which provides safer working environment, cost savings for the process and other important advantages that typical strongly acidic conditions prevent.SUMMARY OF THE INVENTION

[0007] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0008] According to an aspect of the present invention, there is provided a method for manufacturing films and / or coated paper / carton boards from an alkaline cellulose solution via citric acid coagulation.

[0009] This and other aspects, together with the advantages thereof over known solutions are achieved by the present invention, as hereinafter described and claimed.

[0010] The method of the present invention is mainly characterized by what is stated in the characterizing parts of claims 1 and 2.

[0011] Considerable advantages are obtained by means of the invention. Firstly, citric acid is non-toxic and more sustainable chemical compared to commonly used sulphuric acid. By replacing sulphuric acid with citric acid, the process can be made more viable and a safer working environment is achieved. The invented process also allows coating of carbonate pigment (PCC, GCC) coated paper boards, as these tend to dissolve in acidic conditions commonly used.

[0012] Next, the present technology will be described more closely with reference to certain embodiments.EMBODIMENTS

[0013] The present technology provides means for replacing typically used sulphuric acid with more sustainable citric acid, when preparing cellulose films and / or coatings from alkaline cellulose solutions via regeneration coagulation baths and / or liquids.

[0014] FIGURE 1 is a chart disclosing tensile strength (N) of wet films with different regeneration liquids. Plasticizer enhances tensile strength and regeneration liquid has strong influence on tensile.

[0015] FIGURE 2 is a chart disclosing strain at break (%) of wet films with different regeneration liquids. The overall values are high, around 50%.

[0016] FIGURE 3 is a chart disclosing modulus (MPa) of wet films with different regeneration liquids. Modulus appears not to be dependent of plasticizer addition.

[0017] FIGURE 4 is a chart disclosing the effect of regeneration liquid to tensile strength (MPa) of dried films. All three trial points dipped 70 seconds at 5 wt-% glycerol, drying at 23 °C.

[0018] FIGURE 5 is a chart disclosing the effect of regeneration liquid to strain at break (%) of dried films.

[0019] FIGURE 6 is a chart disclosing the effect of regeneration liquid to modulus (MPa) of dried films.

[0020] The present invention is based on using heated and concentrated aqueous citric acid as the regeneration and cross-linking chemical. Citric acid does not act as rapidly as sulphuric acid, whereby more time is allowed for example to the orientation of cellulose molecules. In addition, the by-products formed during the citric acid coagulation are easier to utilize than those formed during sulphuric acid coagulation.

[0021] According to one embodiment of the present invention, the method for manufacturing films and / or coated paper or carton boards from a cellulose-based starting material comprises at least the following steps: dissolving mechano-enzymatically pre-treated virgin or recycled cellulose material in NaOH / ZnO solution thereby obtaining an alkaline cellulose solution, regenerating the alkaline cellulose solution in a coagulation bath that contains heated citric acid in a consistency of at least 35 wt-%, and subjecting the cellulose solution into wet-spinning of films or coating of paper or carton boards.

[0022] According to one embodiment of the present invention, the method for manufacturing films and / or coated paper or carton boards from a cellulose-based starting material, comprises at least the following steps: dissolving mechano-enzymatically pre-treated virgin or recycled cellulose material in NaOH / ZnO solution thereby obtaining an alkaline cellulose solution, regenerating the alkaline cellulose solution by contacting it with a regeneration liquid that contains heated citric acid in a consistency of at least 35 wt-%, andsubjecting the cellulose solution into wet-spinning of films or coating of paper or carton boards.

[0023] According to one embodiment of the present invention, the cellulose material is dissolving grade pulp.

[0024] According to one embodiment of the present invention, the citric acid is heated into a temperature above 20 °C, preferably between 30 and 50 °C and most suitably around 40 °C. In other words, the coagulation bath or the regeneration liquid contains warm citric acid in a temperature above 20 °C, preferably between 30 and 50 °C and most suitably around 40 °C.

[0025] According to one embodiment of the present invention, the consistency of the citric acid in the coagulation bath or in the regeneration liquid is between 35 wt-% and 45 wt-%, preferably around 40 wt-% and most suitably 38 wt-%.

[0026] According to a preferred embodiment of the present invention, the coagulation bath or the regeneration liquid consists of citric acid and water. Commonly used coagulation baths for similar purposes contain sulphuric acid, or citric acid at a low concentration and low temperature with tri-sodium citrate and glycol.

[0027] One embodiment of the present invention is to prepare a coating to at least one surface of the paper or carton board, by leading such surface of the board through the coagulation bath or by contacting it with the regeneration liquid and thereby generating a coating layer having thickness below 20 pm, preferably between 5 and 15 pm and most suitably around 5 pm. Commonly used cellulose-based coatings are thicker, such as above 30 pm, which requires more material and has a negative effect to for example shrinking properties of the coating.

[0028] Another embodiment of the present invention is to prepare a coating to both surfaces of the paper or carton board by sinking the board entirely to the coagulation bath or by contacting the surfaces with the regeneration liquid and thereby generating coating layers having thickness below 20 pm, preferably between 5 and 15 pm and most suitably around 5 pm. This feature evens up the drying tension of the coating, improves water tolerance and protects the board from both sides against for example mineral oil migration, oxygen permission and whatever desired and designed effect.

[0029] In addition, the invented method disclosed herein allows coating of carbonate pigment, such as PCC (precipitated calcium carbonate) and GCC (ground calcium carbonate), coated paper boards. With sulphuric acid-based coagulation baths this is not possible, because the strongly acidic environment forces the pigment(s) to bubble out as CO2. Citric acid has small enough pKa for enabling the pigment(s) to stay in and also preventing other damages to the paper.

[0030] Further embodiment of the present invention is collecting sodium and zinc citrates formed in the coagulation bath or in the regeneration liquid for further use. Sodium citrate is used for example as an acidity regulator in food and drinks and as an emulsifier for oils. Zinc citrate on the other hand is used for example as a food supplement and for dental care products.

[0031] Use of citric acid at a temperature above 20 °C, preferably between 30 and 50 °C and most suitably around 40 °C in a consistency of between 35 wt-% and 45 wt-%, preferably around 40 wt-% and most suitably 38 wt-% for coagulating alkaline cellulose solutions into regenerated cellulose films and / or coatings also belongs to the scope of the present invention.

[0032] Furthermore, it can be concluded that correct selection of regeneration liquid favours machine runnability, produces cost savings and makes machine operation safer when strong acids can be avoided. In addition, the new regeneration liquid according to the present invention allows coating of carbonate containing papers.

[0033] Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed.

[0034] As used herein, a plurality of items, structural elements, compositional elements, and / or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. While the forgoing examples are illustrative of theprinciples of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[0035] The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", that is, a singular form, throughout this document does not exclude a plurality.INDUSTRIAL APPLICABILITY

[0036] The present invention is industrially applicable in for example cellulose- based film manufacturing industry as well as cellulose-based coating and packaging industry.EXAMPLESExample 1A standalone film was manufactured in lab scale from cellulose solution (Biocelsol dissolving method) having solids content of 6.44 % and ball drop viscosity (ball dropping time for 200 mm; Dpipe= Dbaii=l / 8”, 130 mg; modified ASTM D1343-95) of 66 s at 20 °C. The solution was casted on 1.5 mm thick stainless steel ANSI 304 plate using an Ericsson lab coater with a comb having a 400 micron casting gap in room temperature. After casting the stainless steel plate was sunk into 38 wt-% citric acid bath in around 40 °C temperature for two minutes to regenerate the casted solution. After 90 seconds the casted cellulose solution began to detach from steel plate surface and after 120 seconds it was carefully transferred to water bath with running water for ten minutes. After ten minutes the sheet was removed from water bath and placed to milliq water bath in room temperature.Example 2Uncoated carton board with grammage of 150 g / m2was coated using dissolved cellulose solution (Biocelsol dissolving method) in lab scale using Erichsen lab coater and a film applicator with gap clearance of 60 microns producing a calculated dry layer thickness of approximately four microns. Ball drop viscosity (ball dropping time for 200 mm; Dpipe= Dbaii=l / 8”, 130 mg; modified ASTM D1343-95) at 20 °C was measured to be 60 seconds. Solids content was around 6.5 %. After coating carton board was sunk into 38 wt-% citric acid bath in 38 °C temperature for 60 seconds to regenerate the casted solution. After regeneration coated carton board sheet was transferred to room temperature water bath with constant rate of fresh water flowing into the bath. After 10 minutes, the carton board was transferred to drying jig where the sheet was attached from each side to prevent any shrinkage or curling during the drying. Drying was done in standard lab conditions of 23 °C and 50 % of relative air humidity. After drying oxygen barrier and water vapour barrier of coated carton was measured. OTR of the coated carton board was measured 15.1 cc / (m2*day) at 23 °C, 50% RH and 14.6 cc / (m2*day) at 23 °C, 80% RH. Water vapour barrier was measured at 6.5 g / (m2*day) 23 °C, 50% RH.Example 3Regeneration is possible from one side only by means of contacting one side of the formed film or coating layer with regeneration fluid. Any regeneration fluid contact on either side of film is sufficient to regenerate the film thoroughly. In order to improve regeneration efficiency, it is viable to arrange sufficient regeneration fluid flow over the film surface, utilize counter current flow where film moves towards fresh regeneration fluid feed, avoid overflow to over the film side and collect the regeneration fluid after contact for recycle or regenerated the fluid for reuse.In an experiment we set a paper substrate coated with cellulose dope on a tilted surface, angle of which can be adjusted from 1 to 90°. There was arranged a constant flow of regeneration fluid downward on the surface, applying a nozzle through which the regeneration flow was controlled. The sides of the film were protected with tape in order to avoid backside and horizontal wetting from the paper edges. Efficiency of the process ispossible to be affected on numbers of contacts with fresh regeneration liquid and its actual regeneration chemical concentration.In the actual laboratory scale test 150 gsm packaging board was coated with cellulose dope applying Erichsen coater metering with grooved rod of sizes 80 pm with coating speed of 50 mm / s. The evenly coated structure was placed on 30° tilted surface and subjected to flow of regeneration liquid, containing water and 38% citric acid at room temperature, on top side of coated board for 2 min. After the contact with regeneration fluid the coated structure was subjected to flow of water on top side of coated board (5 min). Test was performed in ambient conditions at 24 °C. Regeneration appears to proceed in thickness direction of board while protecting the underlying board from wetting.The sample was dried in oven at 105 °C achieving a sheet of below 6... 12 microns thickness of regenerated cellulose on packaging board. When testing the specimen was observed barrier properties as follows: OTR < 0.2 cc / (m2*day) 50% RH, 23 °C, OTR < 5 cc / (m2day) 80% RH, 23 °C, WVTR < 5 g / (m2day) 50% RH, 23 °C.Industrial implementation example 1Regeneration is possible from one side only by means of contacting one side of the formed film or coating layer with regeneration fluid. Any regeneration fluid contact on either side of film is sufficient to regenerate the film thoroughly. In order to improve regeneration efficiency, it is viable to arrange sufficient regeneration fluid flow over the film surface, utilize counter current flow where film moves towards fresh regeneration fluid feed, avoid overflow to over the film side and collect the regeneration fluid after contact for recycle or regenerated the fluid for reuse.In industrial scale implementation the set-up described in Example 3 can be arranged using different approaches. First option is to arrange the cellulose coated web in downward vertical set up after coating and use gravity driven regeneration liquid flow as thin layer along the coated surface. At desired intervals a fresh regeneration liquid can be brought in contact with coated surface to reach optimal regeneration speed. Coated web can for example be coated and directed downwards several tens of meters with several points where fresh regeneration liquid is brought into contact with coated surface. At lowest point the coated web is turned to opposite vertical direction using a counterclockwise turningroller. Assuming the web direction is from left to right the upwards going part of regeneration section is organized in such manner that regeneration liquid is fed from highest point first followed to desired amount of application points toward the turning roller at lowest point of web. The regeneration liquid is collected at lowest point to recirculation and treatment after it falls due to gravity from lowest point. The control the flow of regeneration liquid on board edges air knife or similar are arranged in such manner that regeneration liquid flow does not enter uncoated areas of the web. The edge areas or the entire web can be supported from the backside of the web by for example backing rollers to be unaffected to air flow from edges guard air knives. Similarly, the washing section after regeneration section can be organized in similar manners with vertical set-up while keeping the web from wetting using directed air flows.Industrial implementation example 2In another industrial scale implementation in the regeneration section a tight contact of paper or carton board to be coated with a large backing roller having adhesion of cellulose coating to backing roller surface can be implemented instead of vertical draws. Upon coating the coating layer is applied wider than the web. Upon regeneration the coating adheres on both coated web and backing roller surface. The transition, wider section on top of backing roller surface prevents the flow of regeneration liquid from both edges and underneath the board in tight contact with the large backing roller. Regeneration liquid is brought to contact with the cellulose coating as surface flow. At desired intervals a fresh regeneration liquid can be brought in contact with coated surface to reach optimal regeneration speed. The regeneration liquid is collected at lowest point to recirculation and treatment after it falls due to gravity from the lowest point. On the second half of the large backing roller the regeneration liquid is fed as counterflow against the backing roll turning direction. At detachment point from the backing roller if there is any remaining cellulose coating the remaining pieces can be scraped away using a doctor blade and finished with showers. Similarly, as in Example 2 the washing section after regeneration section can be organized in similar manner with vertical set-up while keeping the web from wetting using air knives of web edge areas. The vertical set-up in washing stage will assist the web speed.CITATION LISTPatent literatureUS 2016 / 0369456 AlUS 2018 / 215893 Al CN 110042488Non-patent literature:Tu H., Xie K., Ying D., Luo L., Liu X., Chen F., Duan B., Fu Q., Zhang L., Green and Economical Strategy for Spinning Robust Cellulose Filaments, ACS Sustainable Chem. Eng. 2020, 8, 14927-14937.

Claims

CLAIMS:

1. A method for manufacturing films and / or coated paper or carton boards from a cellulose-based starting material, characterized in that the method comprises at least the following steps: dissolving mechano-enzymatically pre-treated virgin or recycled cellulose material in NaOH / ZnO solution thereby obtaining an alkaline cellulose solution, regenerating the alkaline cellulose solution in a coagulation bath that contains heated citric acid in a consistency of at least 35 wt-%, and subjecting the cellulose solution into wet-spinning of films or coating of paper or carton boards.

2. A method for manufacturing films and / or coated paper or carton boards from a cellulose-based starting material, characterized in that the method comprises at least the following steps: dissolving mechano-enzymatically pre-treated virgin or recycled cellulose material in NaOH / ZnO solution thereby obtaining an alkaline cellulose solution, regenerating the alkaline cellulose solution by contacting it with a regeneration liquid that contains heated citric acid in a consistency of at least 35 wt-%, and subjecting the cellulose solution into wet-spinning of films or coating of paper or carton boards.

3. The method according to claim 1 or 2, characterized in that the cellulose material is dissolving grade pulp.

4. The method according to any of the preceding claims, characterized in heating the citric acid into a temperature above 20 °C, preferably between 30 and 50 °C and most suitably around 40 °C.

5. The method according to any of the preceding claims, characterized in that the consistency of the citric acid is between 35 wt-% and 45 wt-%, preferably around 40 wt-% and most suitably 38 wt-%.

6. The method according to any of the preceding claims, characterized in that the coagulation bath or regeneration liquid consists of citric acid and water.

7. The method according to any of the preceding claims, characterized in coating at least one surface of the paper or carton board by leading such surface of the board through the coagulation bath or by contacting it with the regeneration liquid and thereby generating a coating layer having thickness below 20 pm, preferably between 5 and 15 pm and most suitably around 5 pm.

8. The method according to any of the preceding claims, characterized in coating both surfaces of the paper or carton boards by sinking the board entirely into the coagulation bath or by contacting the surfaces with the regeneration liquid and thereby generating coating layers having thickness below 20 pm, preferably between 5 and 15 pm and most suitably around 5 pm.

9. The method according to any of the preceding claims, characterized in coating of carbonate pigment, such as PCC (precipitated calcium carbonate) and GCC (ground calcium carbonate), coated paper boards.

10. The method according to any of the preceding claims, characterized in collecting sodium and zinc citrates formed in the coagulation bath or in the regeneration liquid for further use.

11. Use of citric acid at a temperature above 20 °C, preferably between 30 and 50 °C and most suitably around 40 °C in a consistency of between 35 wt-% and 45 wt-%, preferably around 40 wt-% and most suitably 38 wt-% for coagulating alkaline cellulose solutions into regenerated cellulose films and / or coatings.

12. The use according to claim 11, characterized by the method of any of claims 1 to 10.

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