RMP-containing paperboard and the production thereof

Abstract

The present disclosure provides a multilayered paperboard comprising a top layer, a back layer and a middle layer arranged between the top and the back layer, wherein the middle layer comprises refiner mechanical pulp (RMP) fibers in admixture with at least one other type of fibers. A method of producing a multilayered paperboard is also provided.

Description

RMP-CONTAINING PAPERBOARD AND THE PRODUCTION THEREOFTECHNICAL FIELD

[0001] The present disclosure relates to paperboard and the production thereof.BACKGROUND

[0002] When producing paperboard, it is often desired to obtain a product of high bulk in combination with a fine surface and sufficient strength properties. Therefore, a multilayered design is often used for the paperboard, wherein CTMP, which is known to be a high-bulk and low-strength pulp, is included in the middle layer in admixture with a strength-enhancing pulp.SUMMARY

[0003] Refiner mechanical pulp (RMP) is known to be a pulp of significantly lower bulk than CTMP. This is also confirmed by the present inventor, who shows in example 1 below that after conventional lab sheet forming, RMP from aspen has a bulk of 2.8 cm3 / g, whereas CTMP from aspen, birch and spruce has a bulk of 3.9, 4.3 and 5.1 cm3 / g, respectively.

[0004] The present inventor realized that the conventional lab sheet forming conditions (see e.g., ISO 5269-1:2005 and ISO 5269-2:2004) may not be suited for studying how the pulps will perform in a full-scale process because in the full-scale process, the fiber web is subjected to much higher pressures than in the conventional lab methods. Hence, the above-mentioned sheet formations were repeated using a much higher pressure (40 bar instead of 3 bar). As also shown in example 1, the increased pressure resulted in about 10% lower bulk values for all the tested pulps, which appeared to confirm that RMP should not be used when the objective is to obtain a bulky board product.

[0005] The present inventor then formed lab sheets from some of the pulps used in example 1 in admixture with other pulps to simulate a suitable middle layer (see example 2). Again, the CTMPs resulted in much higher bulk values than the RMP when using the conventional lab method. However, when modifying the lab method by applying a pressure of 40 or 80 bar, it was surprisingly shown that RMP produced bulk values comparable to those obtained with the CTMPs. Hence the present inventor has shown that RMP induces an unexpectedly strong “spring back” qualityin a middle layer composition including it. This finding forms the basis of new paperboard designs.

[0006] Hence the present disclosure provides a multilayered paperboard comprising a top layer, a back layer and a middle layer arranged between the top and the back layer, wherein the middle layer comprises refiner mechanical pulp (RMP) fibers in admixture with at least one other type of fibers. The RMP may be prepared by atmospheric refining or pressurized refining (“PRMP”, see Papermaking Science and Technology, Book 5 Mechanical pulping (1999), Chapter 2, table 1). However, according to the present disclosure the term RMP does not include thermomechanical pulp (TMP). For the avoidance of doubt, RMP also does not include CTMP or CMP. In an embodiment, the RMP is hardwood RMP, such as aspen RMP.

[0007] The at least one other type of fibers may comprise kraft pulp fibers, such as hardwood and / or softwood kraft pulp fibers. In an embodiment, at least 20%, such as at least 30%, by dry weight of the fibers in the middle layer are kraft pulp fibers.

[0008] In an embodiment at least 35%, such as at least 45%, such as at least 50%, by dry weight of the fibers in the middle layer are RMP fibers.

[0009] In an embodiment, at least 90%, such as at least 95%, by dry weight of the fibers in the middle layer are kraft pulp fibers or RMP fibers.

[0010] In an embodiment, the top layer comprises kraft pulp fibers. For example, at least 50%, such as at least 80%, by dry weight of the fibers in the top layer may be kraft pulp fibers. The kraft pulp fibers of the top layer may comprise hardwood kraft pulp fibers and optionally softwood kraft pulp fibers.

[0011] In an embodiment, the back layer comprises kraft pulp fibers. For example, at least 50%, such as at least 80%, by dry weight of the fibers in the back layer may be kraft pulp fibers. The kraft pulp fibers of the back layer may comprise hardwood kraft pulp fibers and optionally softwood kraft pulp fibers.

[0012] The multilayered paperboard may for example have a grammage of 130- 450 g / m2, such as 150-350 g / m2. In the present disclosure, grammage is measured according to ISO 536:2019.

[0013] The middle layer of the multilayered paperboard may for example have a grammage of at least 70 g / m2, such as at least 100 g / m2.

[0014] In an embodiment, the grammage of middle layer is greater than the combined grammage of the top layer and the back layer.

[0015] The present disclosure further provides a method of producing a multilayered paperboard comprising a top layer, a back layer and a middle layer arranged between the top and the back layer, said method comprising:- forming the top layer from a top layer furnish;- forming the back layer from a back layer furnish; and- forming the middle layer from a middle layer furnish comprising refiner mechanical pulp (RMP) and at least one other type of pulp.

[0016] In an embodiment, at least 25%, such as at least 35%, such as at least 40%, by dry weight of the middle layer furnish is RMP.

[0017] In an embodiment, the top layer furnish comprises kraft pulp, such as hardwood kraft pulp and optionally softwood kraft pulp. For example, at least 50%, such as at least 80%, by dry weight of the top layer furnish may be kraft pulp.

[0018] In an embodiment, the back layer furnish comprises kraft pulp, such as hardwood kraft pulp and optionally softwood kraft pulp. For example, at least 50%, such as at least 80%, by dry weight of the back layer furnish may be kraft pulp.

[0019] In an embodiment, a formation of the middle ply furnish comprises supply of a stream of RMP and a stream of broke pulp. The mass flow rate ratio of the RMP stream to the broke pulp stream in the formation of the middle ply furnish may be between 1.5:1 and 1:1.5. The mass flow ratios given herein are based on dry weight.

[0020] The formation of the middle ply furnish may further comprise supply of a stream of kraft pulp, such as a softwood kraft pulp. The ratio of the mass flow of the kraft pulp stream to the combined mass flow of the RMP stream and the broke pulp stream may for example be between 1:20 and 1:4, such as between 1:15 and 1:5.

[0021] In an embodiment, the combined mass flow rate of the RMP stream and the broke pulp stream is at least 50%, such as at least 60%, such as at least 70%, such as at least 75%, such as at least 80%, such as at least 85%, of the mass flow rate of the middle ply furnish.

[0022] In an embodiment, the method comprises forming a multilayered web from the furnishes in a forming section of a paperboard machine and subjecting the multilayered web to wet pressing in a press nip of a pressing section arrangeddownstream the forming section in the paperboard machine, wherein the peak nip pressure of the press nip is at least 30 bar, such as at least 35 bar, such as at least 40 bar. The press nip may for example be a shoe press nip.

[0023] The multilayered web may for example be subjected to wet pressing in at least two press nips in the press section. The least two press nips may be shoe press nips. In an embodiment, the peak nip pressure is at least 30 bar in at least two of the at least two press nips.

[0024] The peak nip pressure in the shoe press nip(s) is preferably below 80 bar.BRIEF DESCRIPTION OF FIGURES

[0025] Figure la is a bar chart showing the bulk of lab sheets formed from different pulps using a wet pressing pressure of 3 bar. Figure lb is a bar chart showing the bulk of lab sheets formed from the same pulps using a wet pressing pressure of 40 bar. Figure tc shows the bulk values of figure lb divided by the bulk values of figure la.

[0026] Figure 2 is a bar chart showing the bulk of lab sheets formed from different pulp mixtures using different wet pressing pressures.

[0027] Figure 3 shows a press section that is advantageously used in an embodiment of the method of the present disclosure.DETAILED DESCRIPTION

[0028] As a first aspect of the present disclosure, there is provided a multilayered paperboard comprising a top layer, a back layer and a middle layer arranged between the top and the back layer.

[0029] The middle layer comprises refiner mechanical pulp (RMP) fibers in admixture with at least one other type of fibers. The RMP fibers may be obtained from hardwood, such as aspen.

[0030] The at least one other type of fibers may comprise kraft pulp fibers, such as hardwood and / or softwood kraft pulp fibers. The kraft pulp fibers in the middle layer may be a consequence of the addition of broke pulp and optionally kraft pulp to the pulp mixture used to form the middle layer. This is further discussed below.

[0031] In an embodiment of the first aspect, at least 20%, such as at least 30%, by dry weight of the fibers in the middle layer are kraft pulp fibers. As an example, the middle layer may comprise at least 10%, such as at least 15%, by dry weight of hardwood fibers and at least 10%, such as at least 15%, by dry weight of softwood fibers.

[0032] The RMP fibers in the middle layer are typically a consequence of the addition of RMP and broke pulp (which typically has the same composition as the multilayered paperboard and hence comprises a substantial amount of RMP fibers) to the pulp mixture used to form the middle layer. This is also discussed below. In an embodiment at least 30%, such as at least 40%, such as at least 45%, such as at least 50%, such as at least 55%, by dry weight of the fibers in the middle layer are RMP fibers.

[0033] The top layer typically comprises kraft pulp fibers. For example, at least 50%, such as at least 80%, by dry weight of the fibers in the top layer may be kraft pulp fibers. The kraft pulp fibers of the top layer may comprise hardwood kraft pulp fibers and optionally softwood kraft pulp fibers. In an embodiment, the top layer comprises hardwood kraft pulp fibers and softwood kraft pulp fibers in a dry weight ratio between 6:1 and 1:1.

[0034] The back layer also typically comprises kraft pulp fibers. For example, at least 50%, such as at least 80%, by dry weight of the fibers in the back layer may be kraft pulp fibers. The kraft pulp fibers of the back layer may comprise hardwood kraft pulp fibers and optionally softwood kraft pulp fibers. In an embodiment, the back layer comprises hardwood kraft pulp fibers and softwood kraft pulp fibers in a dry weight ratio between 6:1 and 1:1.

[0035] The multilayered paperboard may for example have a grammage (including any coating) of 130-450 g / m2, such as 150-400 g / m2. In an embodiment, the grammage, excluding any coating, is 180-300 g / m2.

[0036] The middle layer of the multilayered paperboard typically has a grammage of at least 70 g / m2, such as at least 100 g / m2. Preferably, the grammage of middle layer is greater than the combined grammage of the top layer and the back layer (excluding the weight of any coating).

[0037] In an embodiment, the grammage of the top layer (excluding any coating) is 40-80 g / m2and / or the grammage of the back layer is 30-65 g / m2.

[0038] The density (including any coating) of the multilayered paperboard is typically 610-850 kg / m3. In case of a relatively low grammage (130-250 g / m2), the density is typically 750-850 kg / m3. In case of a relatively high grammage (251-450 g / m2), the density is typically 610-810 kg / m3.

[0039] The density of the middle layer may be 550 kg / m3 or lower, such as 400- 550 kg / m3, such as 420-530 kg / m3.

[0040] In the present disclosure, density is measured according to ISO 534:2011.

[0041] To separate a single layer from the multilayered paperboard and thus facilitate the grammage or density measurement, a FORTUNA Bandknife-Splitting Machine (Type AB 320 E / P) can be used. Such a machine that has been customized for paperboard splitting is commercially available and is used by several major companies in the paperboard field. Alternatively, a surface grinding technique can be used to remove all layers but layer of interest. Such a surface grinding is one of the services that are commercially available at RISE Bioeconomy (formerly Innventia) in Stockholm, Sweden.

[0042] In an embodiment, the multilayered paperboard further comprises at least one pigment-based coating layer provided on the top layer to improve surface and / or printing properties. In an embodiment, the multilayered paperboard comprises two or three pigment-based coating layers provided on the top layer. The at least one pigment-based coating layer typically comprises pigment in the form of calcium carbonate and / or clay and at least one binder. Examples of suitable binders are synthetic binders and optionally modified polysaccharides, such as starch. The total coat weight of the at least one pigment -based coating layer may for example be 10-32 g / m2. The surface of the back layer may also be coated. Such a backside coating typically has a lower coat weight than the least one pigment-based coating layer(s) provided on the top layer.

[0043] As a second aspect of the present disclosure, there is provided a method of producing a multilayered paperboard comprising a top layer, a back layer and a middle layer arranged between the top and the back layer, said method comprising: - forming the top layer from a top layer furnish;- forming the back layer from a back layer furnish; and- forming the middle layer from a middle layer furnish comprising refiner mechanical pulp (RMP) and at least one other type of pulp.

[0044] Embodiments of the multilayered paperboard produced according to the second aspect are described above in connection with the first aspect. Accordingly, the embodiments and examples of the first aspect apply to the second aspect mutatis mutandis.

[0045] In an embodiment, the top layer furnish comprises kraft pulp, such as hardwood kraft pulp and optionally softwood kraft pulp. For example, at least 50%, such as at least 80%, of the top layer furnish may be kraft pulp.

[0046] In an embodiment, the back layer furnish comprises kraft pulp, such as hardwood kraft pulp and optionally softwood kraft pulp. For example, at least 50%, such as at least 80%, of the back layer furnish may be kraft pulp.

[0047] In an embodiment, a formation of the middle ply furnish comprises supply of a stream of RMP, such as a stream of never-dried RMP, at a mass flow rate that that is at least 25%, such as at least 35%, such as at least 40% of the mass flow rate of the middle ply furnish.

[0048] In an embodiment, the formation of the middle ply furnish comprises supply of a stream of broke pulp in addition to the stream of RMP. The mass flow rate ratio (based on dry matter) of the RMP stream to the broke pulp stream in the formation of the middle ply furnish may be between 1.5:1 and 1:1.5, such as between 1.4:1 and 1:1.4.

[0049] The stream of broke pulp typically has essentially the same fiber composition as the multilayered paperboard produced by the method. Accordingly, it will typically comprise substantial amounts of RMP fibers and kraft pulp fibers.

[0050] The formation of the middle ply furnish may further comprise supply of a stream of kraft pulp, such as a softwood kraft pulp. The ratio (based on dry matter) of the mass flow of the kraft pulp stream to the combined mass flow of the RMP stream and the broke pulp stream may for example be between 1:20 and 1:4, such as between 1:15 and 1:5.

[0051] In an embodiment, the combined (dry matter) mass flow rate of the RMP stream and the broke pulp stream is at least 50%, such as at least 60%, such as atleast 70%, such as at least 75%, such as at least 80%, such as at least 85%, of the mass flow rate of the middle ply furnish.

[0052] In an embodiment, the method comprises forming a multilayered web from the furnishes in a forming section of a paperboard machine and subjecting the multilayered web to wet pressing in a press nip of a pressing section arranged downstream the forming section in the paperboard machine. The forming section may comprise a wire for forming a top layer web, a wire for forming a middle layer web, a wire for forming a back layer web and (a) couch(es) for merging the three webs to form the multilayered web. The peak nip pressure of said press nip is preferably at least 30 bar, such as at least 35 bar, such as at least 40 bar. Further, the peak nip pressure may be below 80 bar, such as below 70 bar, such as below 65 bar. The press nip may for example be a shoe press nip, such as a double-felted shoe press nip.

[0053] The press section in which the multilayered web is subjected to pressing typically comprises more than one nip. For example, it may comprise at least two shoe press nips, such as at least two double-felted shoe press nips, in which the peak nip pressure is at least 30 bar, such as 30-70 bar.

[0054] An embodiment of a suitable press section 30 is shown in figure 3. In this press section, a multilayered web formed in an upstream forming section is wet- pressed in a series of three nips: a first double-felted shoe press nip 31 in which the line load is 700-1050 kN / m and the peak nip pressure is 35-55 bar; a second doublefelted shoe press nip 32 in which the line load is 700-1050 kN / m and the peak nip pressure is 35-55 bar; and an unfelted offset (smoothing) press nip 33.EXAMPLESExample 1

[0055] Lab sheets were formed from different pulps (see table 1) using a Dynamic Sheet Former (DSF), which produces oriented (anisotropic) sheets. The formed sheets were first pressed according to the conventional DSF procedure. In this first pressing, the formed sheets were sandwiched between blotting papers and outside these also press felts and the pressure was 2 bar to give a sufficient strength for subsequent step.

[0056] Following the first pressing there was a second pressing step in which a pressure of 3 or 40 bar was applied. To enable the higher pressure, a Fontijne presswith metal plates was used. The pressing time was 5 seconds at peak pressure plus a ramping-up time of 3-4 seconds. After pressing, the sheets were subjected to restrained IR drying. Properties of the dried sheets are presented in table 2 below. As shown in figure 1, the bulk of the RMP was clearly inferior to that of the CTMPs. Further, figure 1 shows that all the mechanical pulps lost about 10% of its bulk when the pressure applied in the second pressing was increased from 3 to 40 bar.

[0057] Table 1. Pulps used in example 1.

[0058] Table 1. Properties of sheets formed in example 1. “Tensile” meansCTMP spruce 40 4.30 14.7 38Example 2

[0059] This example examines fiber blends of a typical middle layer instead of the pure pulps of example 1. The blends are based on assumed middle layer furnishes formed by mixing mechanical pulps with broke pulp and a minor part of kraft pulp. The assumed (uncoated) board structure has the following composition:- Top layer, 56 g / m2, formed from 80% BHKP and 20% BSKP;- Middle layer, 133 g / m2, formed from 45% broke pulp, 10% BSKP and 45% mechanical pulp; and- Back layer, 38 g / m2, formed from 80% BHKP and 20% BSKP.

[0060] Since the broke pulp will have the same fiber composition as the board, the intended middle layer has the following fiber composition (based on dry weight): 19% fibers originating from BSKP; 20% fibers originated from BHKP; and 61% fibers originating from mechanical pulp.

[0061] Accordingly, sheets were prepared according to the method described in example 1 from pulp mixtures having the following composition (based on dry weight): 19% of the BSKP, 20% of the BHKP and 61% of the RMP, spruce CTMP or birch CTMP. The pressure applied in the second pressing step was 3, 40 or 80 bar.Properties of the dried sheets are presented in table 3 below. As shown in figure 2, the type of mechanical pulp greatly affected the bulk when the pressure in the second pressing step was 3 bar. When this pressure was increased to 40 or 80 bar, however, the RMP resulted in almost the same bulk as the CTMPs. This indicates that RMP from a bulk perspective unexpectedly can be as efficient as CTMP when used in full- scale production of a multilayered paperboard.

[0062] Further, it is shown in table 4 below that RMP resulted in higher Z- strengths than the CTMPs. It is also shown that high Z-strength is obtained at a lower pressure for RMP than for CTMP, which indicate that 80 bar may be unnecessarily high for RMP, at least from a Z-strength perspective.

[0063] Table 3. Properties of sheets formed in example 2. “Tensile” means geometrical tensile strength index.

[0064] Table 4. Average Z-strength of sheets formed in example 2 at different pressures.

Claims

CLAIMS1. A multilayered paperboard comprising a top layer, a back layer and a middle layer arranged between the top and the back layer, wherein the middle layer comprises refiner mechanical pulp (RMP) fibers in admixture with at least one other type of fibers, wherein the RMP is hardwood RMP, such as aspen RMP.

2. The multilayered paperboard of claim 1, wherein the hardwood RMP is aspen RMP.

3. The multilayered paperboard of claim 1 or 2, wherein at least 35%, such as at least 45%, by dry weight of the fibers in the middle layer are RMP fibers.

4. The multilayered paperboard of claim 3, wherein at least 50% by dry weight of the fibers in the middle layer are RMP fibers.

5. The multilayered paperboard of any one of the preceding claims, wherein the middle layer further comprises kraft pulp fibers.

6. The multilayered paperboard of claim 5, wherein at least 20% by dry weight of the fibers in the middle layer are kraft pulp fibers.

7. The multilayered paperboard of any one of the preceding claims, wherein the top layer and the back layer comprise kraft pulp fibers.

8. The multilayered paperboard of claim 7, wherein at least 50% by dry weight of the fibers in the top layer and the back layer are kraft pulp fibers.

9. The multilayered paperboard of claim 8, wherein at least 80% by dry weight of the fibers in the top layer and the back layer are kraft pulp fibers.

10. The multilayered paperboard of any one of the preceding claims, wherein the grammage of middle layer is greater than the combined grammage of the top layer and the back layer.

11. A method of producing a multilayered paperboard comprising a top layer, a back layer and a middle layer arranged between the top and the back layer, said method comprising:- forming the top layer from a top layer furnish;- forming the back layer from a back layer furnish; and- forming the middle layer from a middle layer furnish comprising refiner mechanical pulp (RMP) and at least one other type of pulp, wherein the RMP is hardwood RMP, such as aspen RMP.

12. The method of claim 11, further comprising a formation of the middle ply furnish comprising supply of a stream of RMP at a mass flow rate that that is at least 25%, such as at least 35%, such as at least 40% of the mass flow rate of the middle ply furnish.

13. The method of claim 12, wherein the stream of RMP is a stream of never-dried RMP.

14. The method of claim 12 or 13, wherein the formation of the middle ply furnish further comprises supply of a stream of broke pulp.

15. The method of claim 14, wherein the mass flow rate ratio of the RMP stream to the broke pulp stream in the formation of the middle ply furnish is between 1.5:1 and 1:1.5.

16. The method of claim 14 or 15, wherein the stream of broke pulp has essentially the same fiber composition as the multilayered paperboard produced by the method.

17. The method of any one of claims 14-16, wherein the combined mass flow rate of the RMP stream and the broke pulp stream is at least 75% of the mass flow rate of the middle ply furnish.

18. The method of any one of claims 12-17, wherein the formation of the middle ply furnish further comprises supply of a stream of kraft pulp.

19. The method of any one of claims 11-18, further comprising forming a multilayered web from the furnishes in a forming section of a paperboard machine and subjecting the multilayered web to wet pressing in a press nip of a pressing section arranged downstream the forming section in the paperboard machine, wherein the peak nip pressure of the press nip is at least 30 bar, such as at least 35 bar, such as at least 40 bar.

20. The method of claim 19, wherein the peak nip pressure is below 80 bar, such as below 70 bar, such as below 65 bar.

Images