Multi-ply carton for corrugated paperboard

Abstract

The invention relates to a multi-ply containerboard for corrugated board, said multi-ply containerboard comprising a first outer ply, a second outer ply, and at least one intermediate ply sandwiched between the first and second outer ply, wherein the first and second outer ply comprise at least 70 wt-%, based on dry weight, of neutral sulphite semi-chemical (NSSC) pulp, and wherein the intermediate ply comprises at least 30 wt-%, based on dry weight, of unbleached kraft pulp and at least one cellulose-based strength enhancer, and wherein the intermediate ply comprises less than 30 wt-%, based on dry weight, of NSSC pulp.

Description

Technical Field

[0001] This invention relates to a multi-layer containerboard for corrugated cardboard. Background Technology

[0002] Corrugated cardboard (sometimes also called corrugated hardboard or corrugated fiberboard) is a packaging material that can be transformed into different types of packaging solutions. Corrugated cardboard is a fiber-based material made from cellulose fibers. The fibers can be virgin fibers or recycled fibers, such as fibers from used corrugated hardboard or other materials.

[0003] Corrugated board comprises at least one corrugated medium (fluting) and at least one non-corrugated medium (coating) glued to the surface of the corrugated medium. For example, corrugated board may consist of one corrugated medium glued between two layers of linerboard to form a sandwich structure. The sandwich structure can be formed in different ways, such as with single-wall, double-wall, and triple-wall construction, as described in Kirwan M., J., Paper and Paperboard. Packaging Technology, Blackwell Publishing 2005.

[0004] One challenge in corrugated board production is the adhesion between the linerboard and the core paper. Insufficient adhesion leads to delamination, while adding too much adhesive to ensure sufficient adhesion can cause washboarding and curling of the corrugated board. It is crucial that the added adhesive is absorbed into the linerboard and / or the base paper. Delamination occurs if the adhesive is not absorbed by the core / linerboard, and the same happens if it is absorbed excessively into the core / linerboard.

[0005] Different qualities of corrugated board exist, and these can include different types of linerboard and corrugated base paper. Boxboard (also known as CCM or corrugated box material) is a type of board specifically manufactured for producing corrugated board. It comprises both linerboard and corrugated base paper (or corrugated core paper)—two types of paper that make up corrugated board. Because boxboard is primarily made from unbleached natural wood fibers, it is typically brown, although its color can vary depending on the type of wood, pulping process, recycling rate, and impurity content.

[0006] Examples of different types of linerboard are kraft linerboard and testlinerboard. Kraft linerboard is typically produced from kraft pulp, which can be bleached or unbleached, and may comprise one or more layers, with the top layer often optimized for a good printable surface and good moisture resistance. Testlinerboard is primarily produced from recycled corrugated cardboard and is typically manufactured in two layers. Due to the presence of recycled fibers, testlinerboard generally has lower mechanical strength than kraft linerboard, particularly lower burst strength. Kraft linerboard is frequently used for packaging boxes where higher strength properties are required.

[0007] Corrugated core paper is formed from paper or paperboard that is corrugated using a corrugating machine by heating, humidifying and pressing.

[0008] Corrugated core paper is often made from neutral sulfite semi-chemical (NSSC) pulp. NSSC pulp (which is typically made from hardwood species) is known for its exceptional stiffness and high rigidity, making it suitable for corrugated core paper. Neutral sulfite semi-chemical (NSSC) pulping is a well-known, older process in the paper pulping industry. One reason for using NSSC pulping is its typically high yield, often exceeding 60%. In NSSC pulping, the cooking liquor contains sulfites such as Na₂SO₃ or (NH₄)₂SO₃, and alkalis such as NaOH or Na₂CO₃. "Neutral" refers to the pH value of the NSSC cooking liquor, which is typically between 6 and 10. The pulp can be cooked in batch or continuous digesters. Typically, cooking times are between 5 minutes and 3 hours, and cooking temperatures are 160-200°C. NSSC pulp contains a relatively high amount of residual lignin, such as 15-20%, which makes it stiff. The kappa value of NSSC pulp is typically above 70. NSSC pulping is "semi-chemical" because it also includes the mechanical refining of the pulp. Refining can be carried out, for example, at decomposer pressure or at atmospheric pressure using a disc refiner.

[0009] Currently, the strength and mechanical properties of corrugated core paper and base paper are improved by adding a small amount of chemical pulp to mechanical pulp. Typically, 5-15% chemical pulp is added. This, of course, not only increases costs but also leads to a decrease in dewatering speed. One potential approach is to blend semi-chemical pulps such as NSSC with unbleached kraft pulp, although this may result in undesirable optical spots and shading variations, as well as changes in sensory properties.

[0010] The corrugated core paper and the liner paper are attached to each other by applying an adhesive between the corrugated base paper and the liner paper. The liner paper is attached to at least one surface of the corrugated base paper by the adhesive. The adhesive is preferably applied to at least one surface of the grooved corrugated base paper, and then the liner paper is attached to said surface. Any conventional adhesive in the art can be used. The adhesive can be, for example, a glue based on starch that can be extracted from a variety of plants. Some of the most common plants are corn, wheat, barley, rice, potato, cassava, and peas. The starch is preferably natural, i.e., unmodified. The adhesive may also contain water, sodium hydroxide, and boric acid. Other additives may also be added, such as additives to improve wet strength or adhesive bond strength. Additionally, other functional chemicals may be added to improve, for example, moisture resistance or gelling behavior, such as borax, glyoxal, or mixtures thereof.

[0011] A significant challenge in the production of corrugated base paper and corrugated board is moisture resistance. When corrugated board is exposed to moisture, water and water vapor can diffuse through the linerboard and soften the base paper. A common solution to this problem is to increase the basis weight of the corrugated core paper and / or linerboard, but this conflicts with the environmental requirement for lower basis weight materials that consume less raw materials.

[0012] Another solution is to provide a barrier layer on the linerboard to reduce the penetration of water and water vapor. However, this is only a partial solution, as moisture diffusion can still occur on opposite sides or via the edges, thus affecting the mechanical stability of the corrugated board. The barrier layer also increases costs and generally reduces the material's recyclability.

[0013] Corrugated core paper or base paper can also be treated or coated with hydrophobic chemicals, but this typically increases costs and can negatively impact the mechanical properties of the corrugated core paper. High levels of hydrophobic chemicals can also impair the adhesion between the corrugated core paper and the linerboard. In particular, NSSC pulp requires high levels of hydrophobic chemicals to achieve the desired level of water resistance in the finished corrugated core paper.

[0014] New machine concepts and increased machine speeds, coupled with the growing demand for source reduction, have further increased the demand for pulps with improved properties.

[0015] There is still a need for new and improved corrugated core paper and linerboard materials that combine strength, low basis weight, water / moisture resistance, low chemical consumption, low cost, and / or high recycling capacity. Detailed Implementation

[0016] One object of this disclosure is to provide an improved NSSC pulp-based boxboard, preferably a corrugated core for corrugated board, which addresses or improves at least some of the aforementioned problems.

[0017] Another object of this disclosure is to provide a NSSC pulp-based boxboard with improved strength properties, particularly burst strength, which can be manufactured at a high dewatering rate.

[0018] Another object of this disclosure is to provide a cassette paperboard based on NSSC pulp with improved chemical retention, which can be manufactured at a high dewatering rate.

[0019] Another object of this disclosure is to provide an improved method for manufacturing NSSC pulp-based boxboard, preferably for use as corrugated core paper, which addresses or improves at least some of the aforementioned problems.

[0020] The above objectives, as well as other objectives that a person skilled in the art would recognize from this disclosure, are achieved through various aspects of this disclosure.

[0021] This invention is based on the inventive understanding that the strength and mechanical properties of NSSC pulp-based boxboard can be improved by forming a multi-layer boxboard without reducing the dewatering rate. The multi-layer boxboard has an outer layer primarily comprising neutral sulfite semi-chemical (NSSC) pulp and an inner layer comprising at least 30% by weight unbleached kraft pulp (UBKP) and at least one cellulose-based strength enhancer. Without being bound by any particular scientific theory, it is envisioned that placing UBKP and the cellulose-based strength enhancer in the inner layer will not hinder dewatering to the same extent as when UBKP and the cellulose-based strength enhancer are mixed with NSSC pulp. The retention of the cellulose-based strength enhancer in the inner layer is also enhanced by the presence of the unbleached kraft pulp. Furthermore, the use of unbleached kraft pulp in the inner layer also helps reduce the tendency for cracking in the corrugated core.

[0022] According to a first aspect shown herein, a multi-layer carton board for corrugated cardboard is provided, the multi-layer carton board comprising:

[0023] First outer layer,

[0024] The second outer layer, and

[0025] At least one intermediate layer sandwiched between the first and second outer layers,

[0026] The first and second outer layers comprise at least 70% by weight of neutral sulfite semi-chemical (NSSC) pulp based on dry weight, and

[0027] The intermediate layer comprises at least 30% by weight of unbleached kraft pulp (dry weight) and at least one cellulose-based strength enhancer.

[0028] The intermediate layer comprises NSSC pulp based on a dry weight of less than 30% by weight.

[0029] Due to the high NSSC pulp content, the boxboard of this disclosure is particularly suitable as the corrugated base paper for corrugated core paper used in corrugated board. Therefore, in a preferred embodiment, the boxboard is a corrugated core paper. That is, for applications where a high NSSC pulp content is acceptable, the boxboard of this disclosure can also be used as the facing paper in corrugated board.

[0030] The boxboard disclosed herein is a multi-layer boxboard comprising at least three layers: a first outer layer (also called the top layer), an intermediate layer (also called the middle layer), and a second outer layer (also called the back layer). The outer surfaces of the multi-layer boxboard, namely the surfaces of the top layer and the back layer that face away from the intermediate layer, are referred to as the top side and the back side, respectively.

[0031] The boxboard disclosed herein is a multi-layer boxboard comprising at least three layers. This boxboard can be manufactured in a paper machine or paperboard machine suitable for producing multi-layer boxboard. Paper machines or paperboard machines for producing boxboard are well known in the art. Typically, the layout includes a stock handling section, a wet end, a press section, and a drying section, and optionally a calendering section and / or a coating section. At the wet end, the multiple layers can be formed individually using different headboxes and stacked in a wet state, or formed together in a multi-layer headbox. If formed individually, the multiple layers are typically stacked before the press and drying sections of the paper machine.

[0032] In some implementations, the basis weight of each of the first outer layer, the second outer layer, and the intermediate layer is 20-100 g / m³. 2 Within the range, preferably 30-80 g / m 2 Within this range. The total basis weight of multi-layered boxboard is preferably 60-300 g / m². 2 Within the range.

[0033] The multi-layered cartonboard disclosed herein may also include other layers, such as a second intermediate layer, arranged between the second and third layers as intermediate layers. These other layers may contain any combination of fibers or pulp.

[0034] In some embodiments, the multilayer carton board further includes a reinforcing agent or adhesive applied at the interface between the intermediate layer and one or two outer layers. Preferably, the reinforcing agent or adhesive comprises cooked or gelatinized or uncooked starch, or a mixture of cooked or gelatinized or uncooked starch with microfibrillated cellulose (MFC). Preferred reinforcing agents or adhesives are cooked natural starch, or cooked natural starch mixed with microfibrillated cellulose. In some embodiments, the reinforcing agent or adhesive further comprises a crosslinking agent. The crosslinking agent may be, for example, citric acid. In some embodiments, the reinforcing agent or adhesive further comprises an insolubilizer. The insolubilizer may be, for example, an amino resin, glyoxal, or a zirconium salt insolubilizer. Based on dry weight, the amount of reinforcing agent or adhesive applied at the interface between the intermediate layer and one or two outer layers is preferably 0.1-5 g / m². 2 Within the range, more preferably in the range of 0.5-3 g / m 2 Within the range.

[0035] Multi-layered cartonboard is preferably based on NSSC. This means that the cartonboard generally contains at least 50% by weight of NSSC pulp based on dry weight. The outer layers contain a higher content of NSSC pulp than the middle layers.

[0036] "NSSC pulp" is obtained from "NSSC pulping," which is further defined in the background section. NSSC pulp can be hardwood pulp or softwood pulp, or a mixture thereof. NSSC pulp is preferably hardwood pulp or a hardwood / softwood pulp mixture having less than 15% by weight of softwood, preferably less than 10% by weight, and more preferably less than 5% by weight of softwood. Hardwood can be, for example, aspen, alder, poplar, eucalyptus, birch, acacia, or beech. NSSC pulp is preferably prepared by cooking in a cooking liquor containing sulfite (preferably Na₂SO₃ or (NH₄)₂SO₃) and alkali (preferably NaOH or Na₂CO₃). In some embodiments, the yield of NSSC pulp is 60% or more, preferably 65% ​​or more, preferably 70% or more, and more preferably 75% or more. The term "neutral" means that the pH of the NSSC cooking liquor is in the range of 6-10. The cooking time is preferably in the range of 5 minutes to 3 hours. The cooking temperature is preferably in the range of 160-200°C. NSSC pulp may contain a relatively high amount of residual lignin, such as 15-20%. According to ISO 3260, the kappa number of NSSC pulp is generally above 70, preferably above 80, preferably above 95, and more preferably above 100. NSSC pulping is "semi-chemical" because it also includes mechanical refining of the pulp. Refining can be carried out, for example, under degassing pressure or at atmospheric pressure using a disc refiner. Refining can be carried out in one or more steps at the same or different pulp consistency. The first refining step can preferably be carried out at a higher consistency, such as 5-35%, and the second refining step can preferably be carried out at a lower consistency, such as <5%.

[0037] In some embodiments, the NSSC pulp has a water retention value (WRV) in the range of 120-300%, preferably in the range of 120-270%. The WRV value can be determined using a 100-mesh screen according to standard ISO 23714.

[0038] The first and second outer sheets comprise at least 70% by weight of NSSC pulp based on dry weight. In some embodiments, the first and second outer sheets comprise at least 80% by weight, preferably at least 90% by weight of NSSC pulp based on dry weight. The first and second outer sheets may comprise 100% by weight of NSSC pulp, but more commonly, the sheets may also comprise other components such that the first and second outer sheets comprise 95% by weight or less, 90% by weight or less, 85% by weight or less, or 80% by weight or less, or 75% by weight or less of NSSC pulp based on dry weight.

[0039] The non-NSSC pulp portions of the first and second outer layers can contain any type of fiber, such as hardwood and / or softwood fibers, and can contain, for example, chemical pulp, mechanical pulp, thermomechanical pulp, or chemi-thermomechanical pulp (CTMP). The non-NSSC pulp portions of the first and second outer layers can also, for example, contain recycled fibers. For example, the first and second outer layers of this disclosure can consist essentially of NSSC pulp or a mixture of NSSC pulp and recycled fibers. "Recycled fibers" refers to fibrous materials that have previously been incorporated into certain paper or paperboard products. Alternatively, or additionally, the non-NSSC pulp portions of the pulp can, for example, contain reject pulp. For example, the pulp of this disclosure can consist essentially of NSSC pulp and reject pulp. "Reject pulp" refers to pulp prepared by refining screen residue from the pulping process.

[0040] In some embodiments, the first and second outer layers are formed from the same pulp suspension, or from a pulp suspension having the same composition. In some embodiments, the compositions of the first and second outer layers are the same, or nearly the same. In some embodiments, the compositions and basis weights of the first and second outer layers are the same, or nearly the same. Having the same or nearly the same first and second outer layers reduces the problem of deformation of multilayer linerboard when exposed to changes in humidity and temperature.

[0041] The intermediate layer comprises at least 30% by weight of unbleached kraft pulp based on dry weight. In some embodiments, the intermediate layer comprises at least 40% by weight, preferably at least 50% by weight, and more preferably at least 60% by weight of unbleached kraft pulp based on dry weight. The intermediate layer may consist entirely of unbleached kraft pulp and a cellulose-based strength enhancer, but more commonly, the intermediate layer may also contain other components such that the intermediate layer comprises 95% by weight or less, 90% by weight or less, 85% by weight or less, or 80% by weight or less, or 75% by weight or less of unbleached kraft pulp based on dry weight.

[0042] Unbleached kraft pulp, or UBKP, generally refers to unbleached sulfate pulp based on pine and / or spruce. The primary raw material for UBKP is preferably pine, but it may also contain up to 45% by weight of spruce. In some embodiments, UBKP has a kappa value of 55 or higher, preferably 60 or higher, and more preferably 70 or higher, as determined according to SCAN ISOC-1.

[0043] In some embodiments, the unbleached kraft pulp is refined to a Schopper Riegler value in the range of 25-55, preferably in the range of 28-38, as determined according to ISO 5267-1.

[0044] The portion of the intermediate lamella that is not unbleached kraft pulp can contain any type of fiber, such as hardwood and / or softwood fibers, and can contain, for example, chemical pulp, mechanical pulp, thermomechanical pulp, or chemi-thermomechanical pulp (CTMP). The portion of the intermediate lamella that is not unbleached kraft pulp can also, for example, contain recycled fibers. For example, the intermediate lamella of this disclosure can consist essentially of unbleached kraft pulp, or a mixture of unbleached kraft pulp and recycled fibers, and at least one cellulose-based strength enhancer. "Recycled fibers" refers to fibrous materials that have previously been incorporated into certain paper or paperboard products. Alternatively, or additionally, the portion of the pulp that is not unbleached kraft pulp can, for example, contain scrap pulp. For example, the intermediate lamella of this disclosure can consist essentially of unbleached kraft pulp and scrap pulp, and at least one cellulose-based strength enhancer. "Scrape pulp" refers to pulp prepared by refining screen residue from the pulping process.

[0045] The intermediate film layer may further contain NSSC pulp, but in a lower concentration than the first and second outer film layers. The intermediate film layer contains less than 30% by weight of NSSC pulp based on dry weight. Preferably, the intermediate film layer contains less than 20% by weight or less than 10% by weight of NSSC pulp based on dry weight. More preferably, the intermediate film layer contains 5-30% by weight of NSSC pulp, more preferably 10-30% by weight of NSSS pulp, and even more preferably 10-20% by weight of NSSC pulp based on dry weight. In some embodiments, the intermediate film layer does not contain NSSC pulp.

[0046] Due to the high NSSC pulp content in the outer layers, the multilayer carton board still has a high NSSC pulp content overall. In some embodiments, the multilayer carton board contains at least 50% by weight, preferably at least 60% by weight, of NSSC pulp based on dry weight. In some embodiments, the multilayer carton board contains 50-95% by weight, preferably at least 60-95% by weight, of NSSC pulp based on dry weight.

[0047] In some embodiments, the NSSC pulp used in multi-layer carton board is graded NSSC pulp. Graded NSSC pulp is obtained by grading the NSSC pulp starting material into a fine fiber fraction and a coarse fiber fraction. Compared to the starting material, the fine fiber fraction has a higher quantity of shorter and thinner fibers. In other words, the average particle size of the NSSC pulp in the fine fiber fraction is lower than the average particle size of the NSSC pulp in the coarse fiber fraction. The fine fiber fraction can be obtained, for example, by separating the NSSC pulp starting material in a pressure screen to obtain a fraction with shorter and thinner fibers.

[0048] The fine fiber fraction obtained through size grading of NSSC pulp is particularly advantageous for use in the outer layers of multi-layer linerboard because it has less impact on the optical properties of the linerboard compared to the ungraded or coarse fiber fraction in the NSSC pulp. The coarse fiber fraction can be advantageously used in the intermediate layers, where it does not affect the optical properties of the linerboard. In a preferred embodiment, the NSSC pulp used for the first and second outer layers is the fine fiber fraction of graded NSSC. In a preferred embodiment, the NSSC pulp used for the intermediate layers is the coarse fraction of graded NSSC pulp. In some embodiments, the average particle size of the NSSC pulp used in the first and second outer layers is lower than the average particle size of the NSSC pulp used in the intermediate layers.

[0049] The intermediate layer further comprises at least one cellulose-based strength enhancer. The cellulose-based strength enhancer preferably comprises, or is composed of, fine cellulose materials such as highly refined cellulose. Refining or beating cellulose pulp refers to the mechanical treatment and modification of cellulose fibers to provide them with the desired properties. The strength of fiber and paperboard products can be increased by enhancing fiber-to-fiber contact, such as through surface fibrillation. One possibility for increasing the strength of coarser fiber mixtures is to add fine cellulose materials such as cellulose fines (e.g., obtained from white water during web formation), highly refined cellulose, or microfibrillated cellulose (MFC) as strength enhancers.

[0050] In some embodiments, the cellulose-based strength enhancer has a water retention capacity (WRV) value of ≥250%, more preferably ≥300%. Furthermore, the WRV value is preferably ≤500%, more preferably ≤450%, ≤400%, or ≤350%. In some embodiments, the cellulose-based strength enhancer has a WRV value of 250-400%, 250-380%, 250-350%, or 300-350%. The WRV value can be determined using a 200-mesh screen according to standard ISO 23714.

[0051] In some embodiments, the cellulose-based strength enhancer has a Schoper-Riegler (SR) value of 70 or higher, preferably in the range of 70-98, as determined by standard ISO 5267-1.

[0052] In some embodiments, the cellulose-based strength enhancer is selected from cellulose fines, highly refined cellulose having a Schopper Riegler value in the range of 70-90, microfibrillated cellulose (MFC), and mixtures thereof.

[0053] At least one cellulose-based strength enhancer is preferably mixed with UBKP in the intermediate layer. When the strength enhancer is mixed with UBKP, the retention of the cellulose-based strength enhancer in the intermediate layer is improved. The cellulose-based strength enhancer and UBKP can be mixed in a slurry suspension before the formation of the intermediate layer. Alternatively, the cellulose-based strength enhancer and UBKP can be mixed on the wire mesh by adding the cellulose-based strength enhancer to the wet web containing UBKP.

[0054] As used herein, the term cellulose fines generally refers to cellulose particles that are significantly smaller than cellulose fibers. In some embodiments, the term fines, as used herein, refers to fine cellulose particles that can pass through a 200-mesh sieve (equivalent pore diameter 76 μm) of a conventional laboratory grading apparatus (SCAN-CM 66:05). There are two main types of cellulose fines: primary fines and secondary fines. Primary fines are generated during pulping and bleaching, where they are removed from the cell wall matrix through chemical and mechanical treatment. Due to their origin (i.e., complex middle lamella, ray cells, parenchyma cells), primary fines exhibit a lamellar structure with only a small amount of fibrous material. In contrast, secondary fines are generated during pulping.

[0055] As used herein, the term "highly refined cellulose pulp" refers to cellulose pulp that has undergone considerable refining but not to the point that all cellulose pulp would pass through a 200-mesh sieve (equivalent pore diameter 76 μm) of a standard laboratory grading apparatus (SCAN-CM 66:05). As used herein, highly refined cellulose pulp refers to cellulose pulp having a Schoper-Riegler (SR) value of 70 or higher, preferably in the range of 70-90, as determined by standard ISO 5267-1.

[0056] In the context of this patent application, microfibrillated cellulose (MFC) means cellulose particles, fibers or fibrils with a width or diameter of 20 nm to 1000 nm.

[0057] Various methods exist for preparing MFCs, such as single or multiple milling passes, pre-hydrolysis followed by milling of the fibrils, or high-shear disintegration or release. To make MFC manufacturing both energy-efficient and sustainable, one or more pretreatment steps are typically required. Therefore, the cellulose fibers used in the pulp used to produce MFCs can be natural or enzymatically or chemically pretreated, for example, to reduce the amount of hemicellulose or lignin. Cellulose fibers can be chemically modified prior to fibrillation, wherein the cellulose molecules contain functional groups different from (or more than) those found in native cellulose. Such groups particularly include carboxymethyl (CM), aldehydes and / or carboxyl groups (cellulose obtained through N-oxygen-mediated oxidation such as "TEMPO") or quaternary ammonium (cationic cellulose). Modification or oxidation in one of the aforementioned methods makes it easier to disintegrate the fibers into MFCs.

[0058] MFC can be made from wood cellulose fibers derived from either hardwood or softwood. It can also be made from microbial sources, agricultural fibers such as wheat straw pulp, bamboo, bagasse, or other non-wood fiber sources. It can be made from pulp, including pulp derived from virgin fibers (e.g., mechanical pulp, chemical pulp, and / or thermomechanical pulp). It can also be made from waste or recycled paper.

[0059] The amount of cellulose-based strength enhancer in the intermediate layer is higher than in the outer layers. In some embodiments, the amount of cellulose-based strength enhancer in the intermediate layer is higher than in the first and second outer layers, preferably at least twice as high. Having a higher amount of strength enhancer in the intermediate layer and a lower amount in the outer layers provides improved dehydration and better retention of the strength enhancer in the multilayer linerboard. In some embodiments, the strength enhancer is distributed throughout the entire intermediate layer. In some embodiments, the strength enhancer is more distributed on one side of the intermediate layer, or even as a separate “sublayer” of the intermediate layer.

[0060] In some embodiments, the amount of cellulose-based strength enhancer in the intermediate sheet is in the range of 0.5-100 kg / tn, preferably in the range of 1-25 kg / tn, based on the dry weight of the intermediate sheet.

[0061] In some embodiments, the intermediate sheet further comprises starch. The starch in the intermediate sheet may, for example, comprise cooked, gelatinized, or uncooked starch, or a mixture thereof.

[0062] In some embodiments, the intermediate sheet further comprises an internal sizing agent. The internal sizing agent is preferably a hydrophobic sizing agent. In some embodiments, the internal sizing agent is selected from alkyl ketene dimers (AKD), alkenyl succinic anhydride (ASA), rosin, and mixtures thereof.

[0063] In some embodiments, the ratio of the content of the internal sizing agent to the content of the cellulose-based strength enhancer is in the range of 0.1:10 to 10:0.1, more preferably in the range of 0.1:5 to 5:0.1.

[0064] In some embodiments, the internal sizing agent is present in a higher concentration in the intermediate layers than in the first and second outer layers, preferably at least twice as high. Adding an internal sizing agent to linerboard can lead to poor adhesion when the corrugated core is subsequently glued to the linerboard to form the corrugated board. The inventors have discovered that these problems can be reduced by placing the internal sizing agent in the intermediate layers of multi-layer linerboard. A higher amount of internal sizing agent in the intermediate layers and a lower amount in the outer layers provides improved dehydration and better retention of the internal sizing agent in the multi-layer linerboard.

[0065] In some embodiments, the intermediate layer further comprises a polymer-based fixative, preferably a cationic polymer.

[0066] Multilayer carton board may further contain additives such as natural starch or starch derivatives, cellulose derivatives such as sodium carboxymethyl cellulose, fillers, retention and / or drainage chemicals, flocculants, anti-flocculation additives, dry strength additives, softeners, crosslinking aids, sizing chemicals, dyes and colorants, wet strength resins, fixatives, defoaming aids, microbial and slime control aids, or mixtures thereof.

[0067] Placing a larger amount of additives (including cellulose-based strength enhancers and internal sizing agents) with UBKP in the middle layers, rather than distributing the additives throughout the multi-layer linerboard, can provide better glue absorption in the corrugating machine (due to the higher starch absorption of the outer layers). Placing additives, especially internal sizing agents, in the middle layers rather than the outer layers also allows for the use of a larger amount of additives, which can result in less mechanical adsorption creep in the finished corrugated board.

[0068] It has been found that multilayer carton board benefits from not being over-dried. In particular, it has been found that when multilayer carton board is dried to a specific moisture content, better breaking toughness is obtained. In some embodiments, the moisture content of the multilayer carton board is in the range of 3-17% by weight, preferably in the range of 4-14% by weight, and more preferably in the range of 5-15% by weight.

[0069] In some embodiments, the multi-layered boxboard has a breaking toughness index GEOM (ISO / TS17958) of greater than 6 Jm / kg, preferably greater than 7 Jm / kg, and more preferably greater than 8 Jm / kg.

[0070] In some embodiments, the multi-layered carton board has an SCT index GEOM (ISO 9895) of more than 23 Nm / g, preferably more than 24 Nm / g, and more preferably more than 25 Nm / g.

[0071] In some embodiments, at least one outer layer of the multilayer carton board is optimized to provide a good printable surface and good moisture resistance. In some embodiments, the outer layer, at least intended to serve as the outer surface of the corrugated board, is optimized to provide a good printable surface and good moisture resistance. In some embodiments, optimization to provide a good printable surface and good moisture resistance includes surface sizing. In some embodiments, the multilayer carton board is surface-sized. In some embodiments, the multilayer carton board is surface-sized with starch. In some embodiments, the multilayer carton board is surface-sized with a combination of starch and at least one other functional component, said at least one functional component preferably selected from crosslinking agents, reinforcing agents, and hydrophobic sizing agents. The crosslinking agent may be, for example, citric acid. The reinforcing agent may be, for example, microfibrillated cellulose (MFC). The hydrophobic sizing agent may be, for example, alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), SMA (styrene-maleic anhydride), rosin, or mixtures thereof.

[0072] Corrugated board comprises at least one layer of non-corrugated facing paper and at least one layer of corrugated core paper. In the normal production of corrugated board, the corrugated core paper is corrugated and then glued to the facing paper. For example, corrugated board may consist of a single layer of corrugated core paper sandwiched between two layers of facing paper.

[0073] According to a second aspect shown herein, a corrugated board is provided, comprising a multi-layered boxboard as defined with reference to the first aspect as corrugated core paper and / or facing paper.

[0074] Boxboard is preferably used as the corrugated core paper in corrugated board. Due to the high content of NSSC pulp, the boxboard of this disclosure is particularly suitable as the corrugated base paper for corrugated core paper used in corrugated board. Therefore, in the preferred embodiment, the boxboard is corrugated core paper. Any type of facing paperboard can be used.

[0075] In other words, the high content of NSSC pulp is an acceptable application, and the boxboard of this disclosure can also be used as the linerboard in corrugated board.

[0076] According to the third aspect shown herein, a method for manufacturing multi-layer boxboard for use in corrugated cardboard is provided, comprising the following steps:

[0077] a) A first web layer is formed from a first slurry suspension, and the first web layer is dehydrated to obtain a first outer sheet layer;

[0078] b) A second sheet layer is formed from the second slurry suspension, and the second sheet layer is dehydrated to obtain an intermediate sheet layer on the first outer sheet layer;

[0079] c) A third sheet layer is formed from a third slurry suspension, and the third sheet layer is dehydrated to obtain a second outer sheet layer on the intermediate sheet layer;

[0080] The first and third slurry suspensions contain at least 70% by weight of neutral sulfite semi-chemical (NSSC) slurry based on dry weight, and

[0081] The second pulp suspension comprises at least 30% by weight of unbleached kraft pulp (dry weight) and at least one cellulose-based strength enhancer.

[0082] The second slurry suspension contains less than 30% by weight of NSSC slurry based on dry weight.

[0083] The terms first, second, and third web layers do not necessarily indicate the order in which the web layers are formed. Web layers can be formed simultaneously or individually in any order.

[0084] In some implementations, different headboxes and one or more wire meshes are used to form the first, second, and third sheet layers separately and partially dehydrate them, and then they are laminated in a wet state.

[0085] In some implementations, the first, second, and third sheet layers are formed and partially dewatered together using a multi-sheet laminar flow headbox and a single wire mesh. For example, the third sheet layer can be formed and partially dewatered separately, and then laminated with the intermediate sheet layer in a wet state to obtain a second outer sheet layer on the intermediate sheet layer. Alternatively, the third sheet layer can be formed and dewatered together with the second sheet layer.

[0086] In some embodiments, the first and third slurry suspensions contain at least 80% by weight, preferably at least 90% by weight, of NSSC slurry based on dry weight.

[0087] In some embodiments, the compositions of the first and third slurry suspensions are the same. In some embodiments, the compositions and basis weights of the first and third outer sheets are the same or nearly the same.

[0088] In some embodiments, the multi-layered carton board comprises at least 50% by weight, preferably at least 60% by weight, of NSSC pulp based on dry weight.

[0089] In some embodiments, the second pulp suspension comprises at least 40% by weight, preferably at least 50% by weight, and more preferably at least 60% by weight of unbleached kraft pulp based on dry weight.

[0090] In some embodiments, the unbleached kraft pulp is refined to a Schopper Riegler value in the range of 25-55, preferably in the range of 28-38, as determined according to ISO 5267-1.

[0091] In some embodiments, the cellulose-based strength enhancer is selected from cellulose fines, highly refined cellulose having a Schopper Riegler value in the range of 70-90, microfibrillated cellulose (MFC), and mixtures thereof.

[0092] In some embodiments, the amount of cellulose-based strength enhancer in the second slurry suspension is in the range of 0.5-100 kg / tn, preferably in the range of 1-25 kg / tn, based on the dry weight of the second slurry suspension.

[0093] In some embodiments, the second slurry suspension further comprises an internal sizing agent, preferably selected from alkyl ketene dimers (AKD), alkenyl succinic anhydride (ASA), rosin gum, and mixtures thereof.

[0094] In some embodiments, the content of the internal sizing agent in the second slurry suspension sheet is higher than that in the first and third slurry suspensions, preferably at least twice as high.

[0095] In some embodiments, the second slurry suspension further comprises a polymer-based fixative, preferably a cationic polymer.

[0096] The method involves forming multiple webs from a pulp suspension and dewatering them. Methods for forming webs with multiple layers and dewatering them are well known in the art. The carton board can be manufactured in a paper machine or paperboard machine suitable for manufacturing multi-layer carton board. Paper machines or paperboard machines for manufacturing multi-layer carton board are well known in the art. Typically, the mechanical configuration includes a stock handling section, a wet end, a press section and a drying section, and a calendering section and / or a coating section.

[0097] The web is typically formed and dewatered at a wet end, which comprises one or more wire meshes conventional in the art. The multiple sheets can be formed individually and laminated in a wet state using different headboxes, or formed together in a multi-sheet headbox. The web is typically formed in a gap former, but it can also be formed in a fourdrinier type former. If formed individually, the wet sheets are typically laminated or couched together before the press and dry sections of the paper machine. Before lamination, a reinforcing agent or binder can be applied between the intermediate sheets and one or two outer sheets. Preferably, the reinforcing agent or binder comprises cooked or gelatinized or uncooked starch, or a mixture of cooked or gelatinized or uncooked starch with microfibrillated cellulose (MFC). Preferred reinforcing agents or binders are cooked natural starch, or cooked natural starch mixed with microfibrillated cellulose. In some embodiments, the reinforcing agent or binder further comprises a crosslinking agent. The crosslinking agent can be, for example, citric acid. In some embodiments, the reinforcing agent or binder further comprises an insoluble binder. The insoluble binder may be, for example, an amino resin, glyoxal, or a zirconium salt insoluble binder. The reinforcing agent or binder is preferably applied using non-contact deposition techniques such as spraying, foaming, or curtain application, in the form of a paste or aqueous dispersion. Preferably, the solids content of the aqueous dispersion is in the range of 0.5-50% by weight, and more preferably in the range of 1-30% by weight. The amount of reinforcing agent or binder applied is preferably 0.1-5 g / m³. 2 Within the range, more preferably within 0.5-3 g / m 2 Within the range.

[0098] The web typically undergoes further dewatering, which may include, for example, passing the formed multi-layered web through the press section of the paper machine, where the web is passed under high pressure between loaded large rolls to squeeze out as much water as possible. The press section may consist of conventional nip (roll gap) press units and press fabric felt and / or have one or more shoe presses or extended dewatering nip sections. These can operate under different nip or press loads (including different positions, temperatures, and delay times). The press section may be equipped with one or more shoe presses to maximize production. If one or more shoe presses are used, these can operate at press levels above 800 kN / m, for example above 1000 kN / m, for example above 1200 kN / m, or for example above 1450 kN / m. The removed water is typically received by the fabric or felt.

[0099] Following the press section, the multilayer webs can be dried in the drying section. Drying can, for example, involve drying the multilayer webs by passing them around a series of heated drying drums. Drying typically reduces the moisture content to about 1-15% by weight, preferably to about 2-10% by weight. It has been found that multilayer linerboard benefits from not being over-dried. In particular, it has been found that better breaking toughness is obtained when multilayer linerboard is dried to a specific moisture content. In some embodiments, the moisture content of the multilayer linerboard is in the range of 3-17% by weight, preferably in the range of 4-14% by weight, and more preferably in the range of 5-15% by weight.

[0100] The web can be further conditioned with heat and steam and supplied between large corrugated rolls to give the finished corrugated core paper its corrugated shape.

[0101] Unless otherwise stated, the physical properties discussed in this disclosure are determined according to the following criteria:

[0102] Brightness C / 2° +UV ISO 2470-1

[0103] L*C / 2°+UV ISO 5631-1

[0104] a*C / 2°+UV ISO 5631-1

[0105] b*C / 2°+UV ISO 5631-1

[0106] gram weight ISO 536

[0107] Thickness, single sheet ISO 534

[0108] Bulk, single sheet ISO 534

[0109] Air permeability GH ISO 5636-5

[0110] Cobb 30s ISO 535

[0111] Moisture content 50% rh ISO 287

[0112] Scott-Bond Tappi T569

[0113] Tensile strength ISO 1924-3

[0114] Tensile index ISO 1924-3

[0115] Tensile strength md / cd ISO 1924-3

[0116] Elongation ISO 1924-3

[0117] Tensile stiffness ISO 1924-3

[0118] Tensile stiffness index ISO 1924-3

[0119] E modulus ISO 1924-3

[0120] TEA ISO 1924-3

[0121] TEA Index ISO 1924-3

[0122] TEA Index ISO 1924-3

[0123] Fracture toughness ISO / TS 17958

[0124] Fracture toughness index ISO / TS 17958

[0125] Tear resistance ISO 1974

[0126] Tear Index ISO 1974

[0127] SCT ISO 9895

[0128] SCT index ISO 9895

[0129] RCT ISO 12192

[0130] RCT index ISO 12192

[0131] Rupture Index ISO 2759

[0132] Bursting strength ISO 2759

[0133] Unless otherwise stated, the standard method can then be used to determine the physical and mechanical properties in the transverse (cd) and machine direction (md).

[0134] While the invention has been described with reference to various exemplary embodiments, those skilled in the art will understand that various changes can be made and elements can be substituted with equivalents without departing from the scope of the invention. Furthermore, many modifications can be made to adapt particular situations or materials to the teachings of the invention without departing from the basic scope of the invention. Therefore, it is intended that the invention be limited to the specific embodiments disclosed as the best mode contemplated for carrying out the invention, but that the invention encompass all embodiments falling within the scope of the appended claims.

Claims

1. Multi-ply containerboard for corrugated paperboard, the multi-ply containerboard comprising: a first outer ply, a second outer ply, and at least one middle ply sandwiched between the first and second outer plies, wherein the first and second outer plies comprise at least 70 weight-%, based on dry weight, of neutral sulfite semi-chemical (NSSC) pulp, and wherein the middle ply comprises at least 30 weight-%, based on dry weight, of unbleached kraft pulp and at least one cellulose-based strength enhancer, and wherein the middle ply comprises less than 30 weight-%, based on dry weight, of NSSC pulp, wherein the amount of cellulose-based strength enhancer in the middle ply is in the range of 0.5-100 kg / tn, based on the dry weight of the middle ply, wherein the middle ply further comprises an internal sizing agent, and wherein the internal sizing agent is present in the middle ply in a higher amount than in the first and second outer plies.

2. The multi-ply containerboard according to claim 1, wherein the first and second outer plies comprise at least 80 weight-%, based on dry weight, of NSSC pulp.

3. The multi-ply containerboard according to claim 2, wherein the first and second outer plies comprise at least 90 weight-%, based on dry weight, of NSSC pulp.

4. The multi-ply containerboard according to any one of claims 1 to 3, wherein the first and second outer plies are formed from pulp suspensions having the same composition.

5. The multi-ply containerboard according to any one of claims 1 to 3, wherein the middle ply comprises at least 40 weight-%, based on dry weight, of unbleached kraft pulp.

6. The multi-ply containerboard according to claim 5, wherein the middle ply comprises at least 50 weight-%, based on dry weight, of unbleached kraft pulp.

7. The multi-ply containerboard according to claim 6, wherein the middle ply comprises at least 60 weight-%, based on dry weight, of unbleached kraft pulp.

8. The multi-ply containerboard according to any one of claims 1 to 3, wherein the multi-ply containerboard comprises at least 50 weight-%, based on dry weight, of NSSC pulp.

9. The multi-ply containerboard according to claim 8, wherein the multi-ply containerboard comprises at least 60 weight-%, based on dry weight, of NSSC pulp.

10. The multi-ply containerboard according to any one of claims 1 to 3, wherein the unbleached kraft pulp is refined to a Schopper Riegler value in the range of 25-55, determined according to ISO 5267-1.

11. The multi-ply containerboard according to claim 10, wherein the unbleached kraft pulp is refined to a Schopper Riegler value in the range of 28-38, determined according to ISO 5267-1.

12. The multi-ply containerboard according to any one of claims 1 to 3, wherein the cellulose-based strength enhancer is selected from the group consisting of cellulose fines, highly refined cellulose having a Schopper Riegler value in the range of 70-90, microfibrillated cellulose (MFC), and mixtures thereof.

13. The multi-ply linerboard according to any one of claims 1 to 3, wherein the amount of cellulose-based strength enhancer in the middle ply is in the range of 1-25 kg / tn, based on the dry weight of the middle ply.

14. The multi-ply linerboard according to any one of claims 1 to 3, wherein the internal sizing agent is selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), rosin size, and mixtures thereof.

15. The multi-ply linerboard according to any one of claims 1 to 3, wherein the internal sizing agent is at least twice as high in content in the middle ply than in the first and second outer plies.

16. The multi-ply linerboard according to any one of claims 1 to 3, wherein the middle ply further comprises a polymer-based fixing agent.

17. The multi-ply linerboard according to claim 16, wherein the polymer-based fixing agent is a cationic polymer.

18. A corrugated paperboard comprising the multi-ply linerboard according to any one of the preceding claims as a fluting and / or liner.

19. The corrugated paperboard according to claim 18, comprising the multi-ply linerboard as a fluting.

20. A method for manufacturing a multi-ply linerboard for use in a corrugated paperboard, comprising the steps of: a) forming a first web layer from a first pulp suspension and dewatering the first web layer to obtain a first outer ply; b) forming a second web layer from a second pulp suspension and dewatering the second web layer to obtain a middle ply on the first outer ply; c) forming a third web layer from a third pulp suspension and dewatering the third web layer to obtain a second outer ply on the middle ply; wherein the first pulp suspension and the third pulp suspension comprise at least 70 wt% of neutral sulfite semi-chemical (NSSC) pulp, based on dry weight, and wherein the second pulp suspension comprises at least 30 wt% of unbleached kraft pulp and at least one cellulose-based strength enhancer, based on dry weight, and wherein the second pulp suspension comprises less than 30 wt% of NSSC pulp, based on dry weight, wherein the amount of cellulose-based strength enhancer in the middle ply is in the range of 0.5-100 kg / tn, based on the dry weight of the middle ply, wherein the middle ply further comprises an internal sizing agent, and wherein the internal sizing agent is at higher content in the middle ply than in the first and second outer plies.