A method for manufacturing a cast sheet

Abstract

The present invention relates to a method for manufacturing a cast sheet comprising cellulose fibers, which method comprises spreading a thin layer of a slurry comprising cellulose fibers on the surface of a movable endless conveyor belt and drying the slurry for obtaining the cast sheet. An object of the present invention is to provide a method for manufacturing a cast sheet comprising cellulose fibers wherein optimum and well-balanced properties of the cast sheet thus manufactured.

Description

[0001] The present invention relates to a method for manufacturing a cast sheet comprising cellulose fibers, which method comprises spreading a thin layer of a slurry comprising cellulose fibers on the surface of a movable endless conveyor belt and drying the slurry for obtaining the cast sheet. The present invention also relates to a cast sheet based on cellulose fibers.

[0002] Methods for manufacturing a cast sheet are well known in the prior art. For example, EP 1 489 927 in the name of the present applicant discloses a method for manufacturing a top loaded cigarette filler, the method comprising the steps of dosing a slurry composed on a basis of a fraction of fine tobacco and cellulose fibres on a drying conveyor for forming a base layer, spreading over the base layer a fraction of coarse tobacco for forming a top load on the base layer and subsequently subjecting the total to a drying treatment.

[0003] GB 1 203 939 relates to a process of manufacturing a coherent tobacco product, which comprises combining comminuted tobacco with a minor weight proportion, based on the tobacco, of at least one water-insoluble cellulose derivative dissolved in at least one aqueous organic acid of at least 15% by weight of organic acid content and converting the combined tobacco and solution of the cellulose derivative with drying into the coherent tobacco product. A higher wet strength is obtained by incorporating in the formed tobacco product highly hydrated, well-beaten cellulose fibers, wherein the pulp of refined cellulose fibers is prepared from tobacco, particularly stems, or from the usual sources used in the pulp industry.

[0004] GB 2 016899 relates a method of preparing tobacco sheet comprising combining dry comminuted tobacco with a base web composition comprising an aqueous slurry consisting essentially of an adhesive for tobacco and cellulose fiber. to form a castable composition having a solids content of at least 10 percent by weight, casting the composition as a thin sheet, and drying. The formable composition comprises comminuted tobacco, an adhesive agent therefor, and from about 2 to about 12 percent by weight (dry basis) of unrefined short cellulose fiber wherein the cellulose fiber is unrefined hardwood pulp.

[0005] WO 2016 / 050469 relates to a production of homogenized tobacco material comprising a step of adding a cellulose pulp to a grinded blend of tobacco powder, wherein the cellulose pulp includes water and cellulose fibres. In addition to pulping, the cellulose fibres might be subjected to suitable processes such as refining, mechanical pulping, chemical pulping, bleaching, sulphate pulping and combination thereof. The introduction of cellulose fibres in the slurry increases the tensile strength to traction of the web of material, acting as a strengthening agent.

[0006] WO 2016 / 050470 relates to a method of production of a homogenized tobacco material, the method comprising: suspending a binder in an aerosol-former to form a suspension, creating a cellulose pulp from cellulose fibers and water, providing a tobacco powder blend, and combining the suspension of binder in aerosol-former, the cellulose pulp and the tobacco powder blend to form a slurry, wherein the step of forming a pulp with cellulose fibers and water comprises reducing the fiber length of the cellulose fibers by means of grinding in order to obtain a mean fiber length of the cellulose fibers comprised between about 0.2 millimetres and about 4 millimetres.

[0007] WO 2023 / 046774 relates to a process for manufacturing a paper or cardboard sheet from a fibrous suspension, comprising the steps of injecting a polymer into a fibrous suspension, forming a paper or cardboard sheet, and drying the paper or cardboard sheet. In such a paper process a polymer is used, resulting from a functionalization with isocyanates and the addition of micro-cellulose compounds which allows an improvement in the drainage and dry strength properties, while having low metering (low dosage) of micro-cellulose in relation to the pulp, wherein the micro-cellulose compound is selected from nano-fibrillated cellulose, micro- fibrillated cellulose, nano-crystalline cellulose, nano-cellulose.

[0008] US 2021 / 244069 relates to a method for producing reconstituted vegetable strips comprising the steps of milling vegetable materials, mixing cellulose fibers in an intensive mixer, mixing the vegetable material with an agglutinating compound added to the nanocellulose fibers, adding at least one humectant agent and water to the mixture, submitting the mixture to a step of shearing, laminating the mixture for obtaining a continuous strip, drying the vegetable strip obtained from step and cutting and final processing of the dried strip. The step of mixturing cellulose fibers in an intensive mixer may use at least one cellulose fiber, such as a short fiber cellulose, long fiber cellulose, cellulose nanofibers, or other similar cellulose.

[0009] EP 2 639 351 relates to a method for preparation of a cellulose-based product comprising the steps of supplying a pulp stock comprising cellulose fibres, adding one or more retention agents to said pulp stock, adding cellulose nanocrystals (CNC) to said pulp stock, and mixing. The pulp stock may be a dispersion of cellulose fibres in water, with a dry solid content of cellulose fibres in the range of from 0.1 % to 10 %, based on the total weight of the dispersion, wherein the cellulose fibres might be a mixture of different cellulose pulps. A pulp stock may be a result of one or more pre-steps such as dispersion of dry pulp in a dispersion medium and / or refining of pulp per se and / or refining of pulp stock.

[0010] The present inventor found that using one type of fiber in a method for manufacturing a cast sheet, the improvement of a certain aspect often results in the reduction of a different aspect. For instance, the present inventor found that the use of coarse fibers result in an increase of the cross directional tear resistance, but significantly reduces the lengthwise tear resistance. A common solution is to increase the fiber content until both properties reach at least the minimum desired value, which leads to overshooting the level of the other property. By example, a high level of coarse fibers may eventually lead to a sufficient lengthwise tear resistance, but at the same time the cross directional tear resistance will be over engineered.

[0011] A similar effect will be present when manufacturing a sheet with fine fibers, i.e. the sheet thus obtained shows an acceptable lengthwise tear resistance but lacks in cross directional tear resistance. Increasing the content of fine fibers in the sheet will eventually lead to a sufficient cross directional tear resistance but will at the same time lead to an over engineered lengthwise tear resistance.

[0012] In both cases described above, the tensile strength of the sheet with the increased fiber content will be higher than required, and the amount of fiber will often be higher than strictly required, too. This is especially important for sheets containing a characterizing component that needs to deliver a specific property to the sheet, such as taste or aroma. For example, a tobacco sheet is a product where it is often advantageous to maximize the content of tobacco in the sheet. Therefore a sheet with an unnecessary high level of fibres will have an unnecessary low content of tobacco.

[0013] On basis of the above discussion an aim of the present invention is to bring the amount of fibers in a composition to a minimum, while maintaining the required mechanical properties of the sheet.

[0014] An object of the present invention is to provide a method for manufacturing a cast sheet comprising cellulose fibers wherein optimum and well-balanced properties of the cast sheet thus manufactured.

[0015] An object of the present invention is to provide a method for manufacturing a cast sheet comprising cellulose fibers wherein the total amount of fibres is reduced to a minimum while maintaining acceptable mechanical properties of the cast sheet thus manufactured.

[0016] The present invention thus relates to a method for manufacturing a cast sheet comprising cellulose fibers, which method comprises spreading a thin layer of a slurry comprising cellulose fibers on the surface of a movable endless conveyor belt and drying the slurry for obtaining the cast sheet, wherein the cast sheet comprises a combination of at least two differently refined cellulose fibers.

[0017] One or more of the above objects is achieved by the present method. The present inventors surprisingly found that combining cellulose fibers of different coarseness has made it possible to achieve the desired mechanical properties of the sheet, while the required amount of fibers in the sheet is significantly reduced. The present method is particularly suitable for the manufacturing of cast sheet materials to be used in smoke products, such as for example in an aerosol-generating article of the "heat-not-burn" type, wherein an aerosol-forming substrate is heated to a relatively low temperature, in order to form an aerosol but prevent combustion of the tobacco material.

[0018] Maximizing the distinctive or characterizing components in a cast sheet (like tobacco or hemp) is important to give the cast sheet the best possible performance in terms of taste, aroma, visual appearance and smoke or vapor delivery. The present inventors found that the use of a combination of at least two differently refined cellulose fibers will reduce the total amount of fibers required for the technical properties, and thereby increasing the content of the distinctive components resulting in an increase of the organoleptical quality of the final product. Furthermore, the presence of fibers in the slurry requires a sufficient amount of water (or other solvent) to form a good network in the cast slurry prior to drying. The more fibers, the more water is required. Therefore overdosing fibers, which is necessary when only one fiber type is used, will lead to a necessary increase in water usage (lowering the solids content of the slurry) and thereby increasing the cost of drying.

[0019] The present invention opens thus the route to more specialized products where the content of the distinctive components, such as tobacco, hemp, herbs, may be increased as a result of optimizing to the lowest possible content of the structure building ingredients, i.e. the fibers.

[0020] In an example the at least two differently refined cellulose fibers comprise cellulose fibers having different average fiber lengths.

[0021] In an example the difference in average fiber lengths between the at least two differently refined cellulose fibers is at least 200 µm, preferably at least 300 µm.

[0022] In an example the at least two differently refined cellulose fibers comprise cellulose fibers having different fibrillation grades.

[0023] The present inventors assume that when manufacturing cast sheet, as a result of the process, the longer fibers will become more oriented in the machine direction of the sheet, whereas the shorter fibers will have a more random orientation.

[0024] In an example two differently refined cellulose fibers are used in a ratio between 20%:80% and 80%:20%, wherein the percentage is based on the total amount of the two differently refined cellulose fibers.

[0025] The present invention also relates to a cast sheet based on cellulose fibers, wherein the cast sheet comprises a combination of at least two differently refined cellulose fibers.

[0026] In an example of a cast sheet the at least two differently refined cellulose fibers comprise cellulose fibers having different average fiber lengths.

[0027] In an example of a cast sheet the at least two differently refined cellulose fibers comprise cellulose fibers having different fibrillation grades.

[0028] The present invention also relates to the use of a cast sheet as discussed above as an aerosol-generating article of the "heat-not-burn" type.

[0029] Those skilled in the art will understand that they can freely combine all features of the present invention described herein, without departing from the scope of the invention as disclosed.

[0030] Further advantages and features of the present invention are apparent from the following Examples.Examples

[0031] Two different grades of refined cellulose were prepared. Both grades were prepared from so called NBSK pulp (Northern Bleached Softwood Kraft) a commercially available soft wood pulp, and the benchmark pulp used in many paper making processes.

[0032] This base pulp was refined, using a double disc refiner, to obtain two different grades. The grade is identified by the Schopper Riegler number. The Schopper-Riegler test is designed to provide a measure of the rate at which a dilute suspension of pulp may be dewatered. The Schopper degree (°SR) describes the amount of drained suspension and is the degree of the drainage properties of the pulp, which has been diluted in water. The Schopper-Riegler method is according to ISO 5267 / 1.

[0033] A pulp is called SR40 for shortly refined pulp and another pulp is called SR90 for longer refined pulp. The "SR" designation stands for Schopper-Riegler, which is a test used to measure the drainage time of a pulp suspension through a standardized funnel. The number following "SR" (such as SR90) typically represents the pulp's freeness or drainage rate measured in seconds. A lower SR value typically relates to a longer fiber length, and less fibrillated fibers, and a higher SR value typically relates to a shorter, more fibrillated fiber as the microfibrils are formed from the cell walls of the fibers.

[0034] Using an optical fiber tester, the pulps show the following characteristics. SR90: average fiber length: 929 µm, average fiber width:20.7pm.

[0035] SR40: average fiber length: 2082 µm, average fiber width:26.4 µm. Figure 1 shows an image of an SR40 fiber mass. Figure 2 shows an image of an SR90 fiber mass.

[0036] From both Figure 1 and Figure 2 it is clear that a longer refined fiber (SR90) shows much more developed microfibrils than a shorter refined fiber (SR40).

[0037] A longer period of refining, i.e. SR90 is refined longer than SR40, reduces the reduces the fiber length, and increases the formation of micro-fibrils (or crill) on the main fibres. This formation of crill is also visible from the reduction in average fiber width.

[0038] Cast sheets were prepared with a slurry comprising SR40 (Example 1, a comparative example), a slurry comprising SR90 (Example 2, a comparative example) and a slurry comprising equal amounts of SR40 and SR90 (Example 3).

[0039] Different samples were prepared when using composition A as a base composition. In the Table 1 all components are on a dry weight basis. Table 1: components of composition AComponentAmount (w / w)Carboxymethylcellulose15%Cellulose fiber15%Glycerol5%Tobacco powder65%

[0040] The cellulose fiber as a raw material of Composition A was a Northern Bleached Softwood Kraft (NBSK). Two separate batches of refined kraft pulp were prepared from the NBSK. The refining settings for each batch was different.Preparation of Batch SR40

[0041] A batch of long NBSK fibers type was prepared by providing a volume of NBSK in water with a solids content of about 3.5 wt.%. Subsequently, this batch was refined by using a standard method in a double disk refiner resulting in a refining grade with a Schopper Riegler value of about 40. This batch was designated "SR40".Preparation of Batch SR90

[0042] A batch of long NBSK fibers type was prepared by providing a volume of NBSK in water with a solids content of about 3.5 wt.%. Subsequently, this batch was refined by using a standard method in a double disk refiner resulting in a refining grade with a Schopper Riegler value of about 90. This batch was designated "SR90".EXAMPLE 1

[0043] A slurry was prepared according to the components shown in the above Table. In Example 1 the cellulose fiber of composition A was entirely taken from the batch "SR40" as prepared. The solids content of the slurry was 9 wt.%. The slurry was cast as a thin layer on a substrate, and subsequently dried in an oven.EXAMPLE 2

[0044] A slurry was prepared according to the components shown in the above Table. In Example 2 the cellulose fiber of composition A was entirely taken from the batch "SR90" as prepared. The solids content of the slurry was 9 wt.%. The slurry was cast as a thin layer on a substrate, and subsequently dried in an oven.EXAMPLE 3

[0045] A slurry was prepared according to the components shown in the above Table. In Example 3 the cellulose fiber of composition A was for 50% taken from the batch "SR90" and for 50% taken from the batch "SR40". This means that 7.5 wt.% of the cellulose fiber was taken from the batch "SR90" and 7.5 wt.% of the cellulose fiber was taken from the batch "SR40" The solids content of the slurry was 9 wt.%. The slurry was cast as a thin layer on a substrate, and subsequently dried in an oven.

[0046] The properties of the sheets as obtained according to Examples 1-3 are shown in Table 2. Table 2: properties of the sheets as obtained according to Examples 1-3ExampleTensile strength (N / m)Resistance against tearing in the direction perpendicular to the casting directionResistance against tearing in the direction parallel to the casting direction11000highlow21000lowhigh31000highhigh

[0047] The results of Examples 1-2 show that the different fibre qualities used had no significant effect on the tensile strength (Example 1 and 2). In addition, the sheet prepared on basis of the 50 / 50 mix (Example 3) was not significantly different from Example 1 and 2 when the tensile strength is taken into account. However, the sheet according to Example 3 resulted in a strong improvement in resistance against tearing in the direction perpendicular to the casting direction compared to the sheet of Example 2 . Also the sheet of Example 3 showed no notable reduction is resistance against tearing in the direction parallel to the casting direction compared to the sheet of Example 1.

[0048] The results in Table 2 show a synergetic effect of SR40 and SR90 properties, rather than a direct result of the ratios. After all, the total amount of fibers in all three examples is identical, and nevertheless, the resulting sheet from Example 3 shows both an increase in perpendicular tearing resistance compared to Example 2, and an increase in parallel tearing resistance compared to Example 1.

Claims

1. A method for manufacturing a cast sheet comprising cellulose fibers, which method comprises spreading a thin layer of a slurry comprising cellulose fibers on the surface of a movable endless conveyor belt and drying the slurry for obtaining the cast sheet, wherein the cast sheet comprises a combination of at least two differently refined cellulose fibers.

2. A method according to claim 1, wherein the at least two differently refined cellulose fibers comprise cellulose fibers having different average fiber lengths.

3. A method according to claim 2, wherein the difference in average fiber lengths between the at least two differently refined cellulose fibers is at least 200 µm, preferably at least 300 µm.

4. A method according to any one of claims 1-3, wherein the at least two differently refined cellulose fibers comprise cellulose fibers having different fibrillation grades.

5. A method according to any one of claims 1-4, wherein two differently refined cellulose fibers are used in a ratio between 20%:80% and 80%:20%, wherein the percentage is based on the total amount of the two differently refined cellulose fibers.

6. A cast sheet based on cellulose fibers, wherein the cast sheet comprises a combination of at least two differently refined cellulose fibers.

7. A cast sheet according to claim 6, wherein the at least two differently refined cellulose fibers comprise cellulose fibers having different average fiber lengths.

8. A cast sheet according to any one or more of the claims 6-7, wherein the at least two differently refined cellulose fibers comprise cellulose fibers having different fibrillation grades.

9. The use of a cast sheet according to one or more of claims 6-8 as an aerosol-generating article of the "heat-not-burn" type.

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