Multi-ply container board for use in corrugated packaging

Abstract

The present invention relates to a multi-layer container board for use in corrugated board, the multi-layer container board comprising a first outer ply, a second outer ply, and at least one intermediate ply sandwiched between the first and second outer plies, the first and second outer plies comprising at least 70 wt. % neutral sulfite semi-chemical (NSSC) pulp on a dry weight basis, the intermediate ply comprising at least 30 wt. % unbleached kraft pulp and at least one cellulose-based strength agent on a dry weight basis, and the intermediate ply comprising less than 30 wt. % NSSC pulp on a dry weight basis.

Description

[Technical field]

[0001] This invention relates to multi-ply container board for use in corrugated board. [Background technology]

[0002] Corrugated cardboard (sometimes called corrugated cardboard or corrugated fiberboard) is a packaging material that can be converted into various types of packaging solutions. Corrugated cardboard is a fibrous substrate made from cellulose fibers. The fibers can be virgin or recycled, such as fibers derived from post-consumer cardboard or other materials.

[0003] Corrugated board comprises at least one medium (fluting) and at least one flat base paper (liner or linerboard) bonded to the surface of the medium. For example, corrugated board may consist of a layer of fluting bonded between two layers of liner to form a sandwich structure. The sandwich structure can be formed in various formats such as single, double, and triple wall, as described for example in Kirwan M., J., Paper and Paperboard. Packaging Technology, Blackwell Publishing 2005.

[0004] One challenge when producing corrugated board is the adhesion between the liner and the fluting. Too little adhesion can cause delamination, and adding too much adhesive to ensure adequate adhesion can cause washboarding and curling of the board. It is important that the glue added is optimally absorbed into the liner and / or corrugating medium. If the adhesive does not adhere to the fluting / liner, delamination will occur, and if too much is absorbed into the fluting / liner, the same can happen.

[0005] There are various types of corrugated board qualities, which may contain different types of liner and medium. Container board (also known as CCM or corrugated case stock) is a type of paperboard manufactured specifically for the production of corrugated board and contains both linerboard and medium (or fluting), the two types of paper that make up corrugated board. Container board is generally brown in color because it is made primarily from natural unbleached wood fibers, although its color can vary depending on the wood species, pulping process, recycling rate, and impurity content.

[0006] Examples of different types of liners are kraftliners and testliners. Kraftliners are typically produced from kraft pulp, which can be bleached or unbleached, and may contain one or more layers / plies, where the top layer / ply is often optimized to provide a good print surface and good moisture resistance. Testliners are primarily produced from recycled corrugated cardboard and are usually manufactured with two layers / plies. Due to the presence of recycled fibers, testliners may typically have lower mechanical strength than kraftliners, especially lower burst strength. Kraftliners are often used for packaging boxes with higher demands on strength properties.

[0007] Fluting is formed from paper or paperboard that has been corrugated using a corrugator with the application of heat, moisture and pressure.

[0008] Fluting is often prepared from Neutral Sulfite Semi-Chemical (NSSC) pulp. NSSC pulp, usually made from hardwood species, is noted for its exceptional stiffness and high stiffness, making it suitable for use in fluting. Neutral Sulfite Semi-Chemical (NSSC) pulping is an old process well known in the field of paper pulping. One of the reasons for using NSSC pulping is the high yield, usually above 60%. In NSSC pulping, the cooking liquor contains a sulfite salt such as Na2SO3 or (NH4)2SO3 and a base such as NaOH or Na2CO3. By "neutral" we mean that the pH of the NSSC cooking liquor is generally between 6 and 10. The pulp can be cooked in a batch or continuous cooker. Typically, the cooking time is between 5 minutes and 3 hours, and the cooking temperature is between 160 and 200°C. NSSC pulp contains a relatively high amount of residual lignin, such as 15-20%, which makes NSSC pulp hard. The Kappa number of NSSC pulp is typically above 70. NSSC pulping is "semi-chemical" in the sense that it also includes mechanical refining of the pulp. Refining can be done, for example, using a disc refiner at digester pressure or atmospheric pressure.

[0009] Currently, the strength and mechanical properties of the fluting and medium are improved by adding small amounts of chemical pulp to the mechanical pulp. Typically, 5-15% chemical pulp is added, which of course increases the cost but slows down the dewatering rate. One possible method is to mix a semi-chemical pulp such as NSSC with unbleached kraft pulp, but this can result in undesirable light mottling and color changes, as well as changes in organoleptic properties.

[0010] The fluting and liner are attached to one another by placing an adhesive between the core and the liner. The liner is attached to at least one surface of the core by the adhesive. The adhesive is preferably applied to at least one surface of the fluted core, and then the liner is attached to said surface. Any adhesive conventional in the art can be used. The adhesive can be, for example, a starch-based glue that can be extracted from a wide variety of plants. Some of the most common plants are corn, wheat, barley, rice, potato, tapioca, and pea. The starch is preferably natural, i.e., the starch is not modified. The adhesive can also include water, sodium hydroxide, and boric acid. Other additives may be added, such as additives that improve wet strength or adhesive bond strength. Also, other functional chemicals, such as borax, glyoxal, or mixtures thereof, can be added, for example to improve moisture resistance or gelling behavior.

[0011] One significant challenge when making corrugated board and medium is resistance to moisture. When corrugated board is exposed to moisture, water and water vapor can diffuse through the liner and soften the medium. Common solutions to this problem are fluting and / or increasing the basis weight of the liner, but this conflicts with environmental imperatives that call for lower basis weight materials that consume less raw materials.

[0012] Another solution is to provide a barrier layer in the liner to reduce water and water vapor transmission. However, this is only an incomplete solution, since moisture diffusion can still occur on the other side or through the edges, thus affecting the mechanical stability of the board. Barrier layers also increase costs and usually reduce the recyclability of the material.

[0013] The flutings or medium may also be treated or coated with hydrophobizing chemicals, but this generally increases cost and may also have a negative effect on the mechanical properties of the fluting. High levels of hydrophobizing chemicals may also impair the adhesion of the fluting to the liner. In particular, NSSC pulp requires high levels of hydrophobizing chemicals to obtain the required level of water resistance in the finished fluting.

[0014] New machine concepts and increased machine speeds coupled with an increased desire to conserve resources have further increased the need for pulps with improved properties.

[0015] There remains a need for new and improved fluting and liner materials that combine strength, low basis weight, water / moisture resistance, low chemical consumption, low cost, and / or high recyclability. Summary of the Invention

[0016] It is an object of the present disclosure to provide an improved NSSC pulp based container board, preferably for use in fluting for corrugated board, that overcomes or ameliorates at least some of the problems discussed above.

[0017] It is a further object of the present disclosure to provide an NSSC pulp based container board with improved strength properties, particularly burst strength, that can be produced at high dewatering rates.

[0018] It is a further object of the present disclosure to provide a NSSC pulp based container board with improved chemical retention that can be produced at high dewatering rates.

[0019] It is still a further object of the present disclosure to provide an improved process for producing container board based on NSSC pulp, preferably for use in fluting for corrugated board, which overcomes or ameliorates at least some of the problems mentioned above.

[0020] The above objectives, as well as other objectives that will be appreciated by those skilled in the art in light of this disclosure, are accomplished by various aspects of the present disclosure.

[0021] The present invention is based on the inventive realization that the strength and mechanical properties of NSSC pulp-based container board can be improved without reducing the dewatering rate by forming a multi-layer container board having outer plies consisting mainly of neutral sulfite semi-chemical (NSSC) pulp and an intermediate ply containing at least 30 wt% unbleached kraft pulp (UBKP) and at least one cellulose-based strength improver. Without being bound to any particular scientific theory, it is believed that placing the UBKP and cellulose-based strength improver in the intermediate ply does not hinder dewatering to the same extent as when the UBKP and cellulose-based strength improver are mixed with the NSSC pulp. Retention of the cellulose-based strength improver in the intermediate ply is also enhanced by the presence of unbleached kraft pulp. The use of unbleached kraft pulp in the intermediate ply can also help reduce cracking tendency in the fluting. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] According to a first aspect shown herein, there is provided a multi-layer container board for use in corrugated board, said multi-layer container board comprising: A first outer ply; a second outer ply; and at least one intermediate ply sandwiched between the first and second outer plies; Including, the first and second outer plies comprise at least 70 wt. % neutral sulfite semi-chemical (NSSC) pulp on a dry weight basis; the intermediate ply comprises at least 30 wt. % unbleached kraft pulp on a dry weight basis and at least one cellulose-based strength agent; A multi-ply container board is provided, wherein the middle ply comprises less than 30 wt. % NSSC pulp on a dry weight basis.

[0023] Due to the high NSSC pulp content, the container board of the present disclosure is particularly useful as a fluting medium for use in corrugated board. Thus, in a preferred embodiment, the container board is fluted. As mentioned above, the container board of the present disclosure may be used as a liner in corrugated board for applications where a high NSSC pulp content can be tolerated.

[0024] The container paperboard of the present disclosure is a multi-ply container paperboard including at least three plies, a first outer ply (also referred to as a top ply), an intermediate ply (also referred to as a center ply), and a second outer ply (also referred to as a back ply). The outer surfaces of the multi-ply container paperboard, i.e., the surfaces of the top and back plies away from the intermediate ply, are referred to as the top and bottom surfaces, respectively.

[0025] The container board of the present disclosure is a multi-layer container board including at least three plies. The container board can be manufactured on a paper or board machine adapted to manufacture multi-layer container board. Paper or board machines for making container board are well known in the art. Typically, the machine layout includes a stock processing section, a wet end section, a press and dry section, and optionally a calender and / or coating section. In the wet end section, the plies may be formed separately using different headboxes and laminated wet, or may be formed together in a multi-layer headbox. If formed separately, the plies are typically laminated before the press and dry section of the paper machine.

[0026] In some embodiments, the first outer ply, the second outer ply, and the intermediate ply each have a basis weight of 20 to 100 g / m 2 in the range of 30 to 80 g / m 2 The total basis weight of the multi-layer container board is preferably in the range of 60 to 300 g / m 2 The range is.

[0027] The multi-ply container board of the present disclosure may also include additional plies, such as a second intermediate ply disposed as a central ply between the second and third plies. Such additional plies may include any type of fiber or pulp combination.

[0028] In some embodiments, the multi-ply container board further comprises a reinforcing or adhesive applied to the interface between the intermediate ply and one or both of the outer plies. Preferably, the reinforcing or adhesive comprises digested or gelatinzed or uncooked starch, or a mixture of digested or gelatinzed or uncooked starch and microfibrillated cellulose (MFC). A preferred reinforcing or adhesive is digested native starch or digested native starch mixed with microfibrillated cellulose. In some embodiments, the reinforcing or adhesive further comprises a cross-linking agent. The cross-linking agent may be, for example, citric acid. In some embodiments, the reinforcing or adhesive further comprises an insolubilizer. The insolubilizer may be, for example, an amino resin, glyoxal, or a zirconium salt insolubilizer. The amount of reinforcing or adhesive applied to the interface between the intermediate ply and one or both of the outer plies is preferably from 0.1 to 5 g / m on a dry weight basis. 2 More preferably, it is in the range of 0.5 to 3 g / m 2 The range is.

[0029] The multi-ply container board is preferably based on NSSC, meaning that the container board generally preferably contains at least 50 wt% NSSC pulp on a dry weight basis, with the outer plies containing a higher NSSC pulp content than the middle plies.

[0030] "NSSC pulp" is obtained by "NSSC pulping", as such is 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 containing less than 15 wt% softwood, preferably less than 10 wt% softwood, more preferably less than 5 wt% softwood. Hardwood can be, for example, aspen, alder, poplar, eucalyptus, birch, acacia, or beech. NSSC pulp is preferably prepared by cooking with a cooking liquor containing sulfite, preferably Na2SO3 or (NH4)2SO3, and a base, preferably NaOH or Na2CO3. In some embodiments, the yield from NSSC pulping is greater than 60%, preferably greater than 65%, preferably greater than 70%, more preferably greater than 75%. The term "neutral" means that the pH of the NSSC cooking liquor is in the range of 6 to 10. 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%. The Kappa number of NSSC pulp is typically above 70 according to ISO 3260, preferably above 80, preferably above 95, more preferably above 100. NSSC pulping is "semi-chemical" in the sense that it also includes mechanical refining of the pulp. Refining may be carried out, for example, using a disc refiner at digester pressure or at atmospheric pressure. Refining may be carried out in one or more steps at the same or different pulp consistencies. The first refining step may preferably be carried out at a higher consistency, such as 5-35%, and the second refining step may preferably be carried out at a lower consistency, <5%.

[0031] 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 may be determined according to ISO standard 23714 using a 100 mesh wire.

[0032] The first and second outer plies comprise at least 70 wt% NSSC pulp on a dry weight basis. In some embodiments, the first and second outer plies comprise at least 80 wt% NSSC pulp on a dry weight basis, preferably at least 90 wt% NSSC pulp. The first and second outer plies may comprise 100 wt% NSSC pulp, but more typically the plies may also comprise other ingredients such that the first and second outer plies comprise up to 95 wt%, up to 90 wt%, up to 85 wt%, up to 80 wt%, or up to 75 wt% NSSC pulp on a dry weight basis.

[0033] The non-NSSC pulp portions of the first and second outer plies may include any type of fiber, such as hardwood and / or softwood fibers, for example, chemical pulp, mechanical pulp, thermomechanical pulp, or chemi-thermomechanical pulp (CTMP). The non-NSSC pulp portions of the first and second outer plies may also include, for example, recycled fibers. For example, the first and second outer plies of the present disclosure may consist essentially of NSSC pulp or a mixture of NSSC pulp and recycled fibers. By "recycled fibers" it is meant fibrous material that has previously been incorporated into some paper or board product. Alternatively, or as a complement, the non-NSSC pulp portions may include, for example, reject pulp. For example, the pulp of the present disclosure may consist essentially of NSSC pulp and reject pulp. By "reject pulp" it is meant pulp prepared by refining screen rejects from a pulping process.

[0034] In some embodiments, the first and second outer plies are formed from the same pulp suspension or from pulp suspensions having the same composition. In some embodiments, the composition of the first and second outer plies is the same or nearly the same. In some embodiments, the composition and basis weight of the first and second outer plies is the same or nearly the same. Having the same or nearly the same first and second outer plies reduces problems with deformation of the multi-layer container board when exposed to fluctuations in humidity and temperature.

[0035] The intermediate ply comprises at least 30 wt% unbleached kraft pulp on a dry weight basis. In some embodiments, the intermediate ply comprises at least 40 wt%, preferably at least 50 wt%, more preferably at least 60 wt% unbleached kraft pulp on a dry weight basis. The intermediate ply may consist entirely of unbleached kraft pulp and cellulose-based strength additives, but more typically the intermediate ply may also comprise other components such that the intermediate ply comprises up to 95 wt%, up to 90 wt%, up to 85 wt%, up to 80 wt%, or up to 75 wt% unbleached kraft pulp on a dry weight basis.

[0036] 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 may also contain up to 45 wt% spruce. In some embodiments, the UBKP has a Kappa number greater than 55, preferably greater than 60, and more preferably greater than 70, as determined according to SCAN ISO C-1.

[0037] In some embodiments, the unbleached kraft pulp is refined to a Schopper Riegler number in the range of 25 to 55, preferably in the range of 28 to 38, as determined according to ISO 5267-1.

[0038] The non-unbleached kraft pulp portion of the intermediate ply may include any type of fiber, such as hardwood and / or softwood fibers, for example, chemical pulp, mechanical pulp, thermomechanical pulp, or chemi-thermomechanical pulp (CTMP). The non-unbleached kraft pulp portion of the intermediate ply may also include, for example, recycled fibers. For example, the intermediate ply of the present disclosure may 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 some paper or board product. Alternatively, or as a complement, the non-unbleached kraft pulp portion of the pulp may include, for example, reject pulp. For example, the intermediate ply of the present disclosure may consist essentially of unbleached kraft pulp and reject pulp and at least one cellulose-based strength enhancer. "Reject pulp" refers to pulp prepared by refining screen rejects from a pulping process.

[0039] The intermediate ply may further comprise NSSC pulp, but at a lower content than the first and second outer plies. The intermediate ply comprises less than 30 wt% NSSC pulp on a dry weight basis. Preferably, the intermediate ply comprises less than 20 wt% or less than 10 wt% NSSC pulp on a dry weight basis. It may be preferred for the intermediate ply to comprise 5-30 wt% NSSC pulp, preferably 10-30 wt% NSSC pulp, even more preferably 10-20 wt% NSSC pulp on a dry weight basis. In some embodiments, the intermediate ply does not comprise NSSC pulp.

[0040] Due to the high NSSC pulp content in the outer plies, the multi-layer container board as a whole also has a high NSSC pulp content. In some embodiments, the multi-layer container board comprises at least 50 wt% NSSC pulp on a dry weight basis, preferably at least 60 wt%. In some embodiments, the multi-layer container board comprises 50-95 wt% NSSC pulp on a dry weight basis, preferably at least 60-95 wt%.

[0041] In some embodiments, the NSSC pulp used in the multi-layer container board is a fractionated NSSC pulp. The fractionated NSSC pulp is obtained by size fractionation of 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 amount of shorter, finer 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 with a pressure screen to obtain a fraction of shorter, finer fibers.

[0042] The fine fiber fraction obtained by size fractionation of NSSC pulp is particularly advantageous for use in the outer plies of multi-layer container boards because it has less effect on the optical properties of the liner compared to unfractionated or coarse fiber fractions of NSSC pulp. The coarse fiber fraction may be advantageously used in the intermediate ply where it does not affect the optical properties of the liner. In preferred embodiments, the NSSC pulp used in the first and second outer plies is a fine fiber fraction of fractionated NSSC. In preferred embodiments, the NSSC pulp used in the intermediate ply is a coarse fraction of fractionated NSSC pulp. In some embodiments, the average particle size of the NSSC pulp used in the first and second outer plies is lower than the average particle size of the NSSC pulp used in the intermediate ply.

[0043] The intermediate ply further comprises at least one cellulose-based strength enhancer. The cellulose-based strength enhancer preferably comprises or consists of fine cellulose material, such as highly refined cellulose. Refining, or beating, of cellulose pulp refers to the mechanical treatment and modification of cellulose fibers to impart desired properties to them. The strength of the fibers and paperboard products can be increased by increasing the fiber-fiber contact, for example by surface fibrillation. One possibility to increase the strength of coarser fiber mixtures is to add fine cellulose materials as strength enhancers, such as cellulose fines obtained from white water during web formation, highly refined cellulose, or microfibrillated cellulose (MFC).

[0044] In some embodiments, the cellulose-based strength enhancer has a water retention (WRV) value of ≧250%, more preferably ≧300%. Furthermore, the WRV value is preferably ≦500%, more preferably ≦450% or ≦400% or ≦350%. In some embodiments, the cellulose-based strength enhancer has a WRV value of 250-400%, or 250-380%, or 250-350%, or 300-350%. The WRV value may be determined using 200 mesh wire according to ISO standard 23714.

[0045] In some embodiments, the cellulose-based strength enhancer has a Schopper-Riegler (SR) number greater than 70, preferably in the range of 70-98, as determined by ISO standard 5267-1.

[0046] In some embodiments, the cellulose-based strength agent is selected from the group consisting of cellulose fines, highly refined cellulose having a Schopper Riegler number in the range of 70 to 90, microfibrillated cellulose (MFC), and mixtures thereof.

[0047] At least one cellulose-based strength enhancer is preferably mixed with the UBKP in the intermediate ply. Retention of the cellulose-based strength enhancer in the intermediate ply is improved when the strength enhancer is mixed with the UBKP. The cellulose-based strength enhancer and the UBKP can be mixed in the pulp suspension prior to forming the intermediate ply. Alternatively, the cellulose-based strength enhancer and the UBKP can be mixed on the wire by adding the cellulose-based strength enhancer to a wet web containing the UBKP.

[0048] The term cellulose fines as used herein generally refers to cellulose particles that are significantly smaller in size than cellulose fibers. In some embodiments, the term fines as used herein refers to fine cellulose particles that can pass through a 200 mesh screen (76 μm equivalent hole diameter) of a conventional laboratory fractionator (SCAN-CM 66:05). There are two main types of fiber fines: primary and secondary fines. Primary fines are generated during pulping and bleaching, where they are removed from the cell wall matrix by chemical and mechanical treatments. As a result of their origin (i.e., complex intercellular layers, ray cells, parenchyma cells), primary fines exhibit a flake-like structure that shares only small amounts of small fiber material. In contrast, secondary fines are generated during the refining of the pulp.

[0049] As used herein, the term highly refined cellulose pulp refers to cellulose pulp that has been subjected to significant refining, but not enough that all of the cellulose pulp passes through a 200 mesh screen (76 μm equivalent hole diameter) of a conventional laboratory fractionator (SCAN-CM 66:05). As used herein, the term highly refined cellulose pulp refers to cellulose pulp having a Schopper-Riegler (SR) number greater than 70, preferably in the range of 70 to 90, as determined by ISO Standard 5267-1.

[0050] Microfibrillated cellulose (MFC) in this patent application means cellulose particles, fibres or fibrils having a width or diameter between 20 nm and 1000 nm.

[0051] There are various methods to make MFC, such as single or multiple pass refining, prehydrolysis followed by refining or high shear disintegration or release of fibrils. One or several pretreatment steps are usually required to make the production of MFC energy efficient and sustainable. The cellulose fibers of the pulp used in producing MFC in this way may be natural or may be enzymatically or chemically pretreated, for example to reduce the amount of hemicellulose or lignin. The cellulose fibers may be chemically modified before fibrillation, where the cellulose molecules contain functional groups other than (or additional to) those found in the original cellulose. Such groups include, among others, carboxymethyl (CM), aldehyde and / or carboxyl groups (cellulose obtained by N-oxyl mediated oxidation, e.g. "TEMPO"), or quaternary ammonium (cationic cellulose). After being modified or oxidized by one of the above methods, it is easier to break down the fibers into MFC.

[0052] MFCs can be produced from wood cellulose fibres, both hardwood or softwood fibres, they can also be made from microbial sources, agricultural fibres such as wheat straw pulp, bamboo, bagasse or other non-wood fibre sources, they can be made from pulps derived from virgin fibres, including mechanical, chemical and / or thermomechanical pulps, and they can also be made from shredded or recycled paper.

[0053] The amount of cellulose-based strength enhancer in the middle ply is higher in the middle ply than in the outer plies. In some embodiments, the amount of cellulose-based strength enhancer is higher in the middle ply than in the first and second outer plies, preferably at least two times higher. Having a higher amount of strength enhancer in the middle ply and a lower amount in the outer plies provides improved drainage and better retention of strength enhancer in the multi-layer container board. In some embodiments, the strength enhancer is distributed throughout the middle ply. In some embodiments, the strength enhancer is distributed more on one side of the middle ply, or even as a separate "sub-ply" of the middle ply.

[0054] In some embodiments, the amount of cellulose-based strength enhancer in the intermediate ply is in the range of 0.5 to 100 kg / tn, preferably in the range of 1 to 25 kg / tn, based on the dry weight of the intermediate ply.

[0055] In some embodiments, the intermediate ply further comprises starch. The starch of the intermediate ply may comprise, for example, cooked or gelatinized or uncooked starch, or a mixture thereof.

[0056] In some embodiments, the intermediate ply further comprises an internal size, preferably a hydrophobizing size, hi some embodiments, the internal size is selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), rosin size, and mixtures thereof.

[0057] 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.

[0058] In some embodiments, the content of the internal size is higher in the middle ply than in the first and second outer plies, preferably at least two times higher. Adding an internal size to the container board can cause poor adhesion problems when the flutes are subsequently attached to the liner to form the corrugated board. The inventors have found that these problems can be reduced by placing the internal size in the middle ply of the multi-layer container board. Having a higher amount of internal size in the middle ply and a lower amount in the outer plies provides improved drainage and better retention of the internal size within the multi-layer container board.

[0059] In some embodiments, the intermediate ply further comprises a solidifying agent based on a polymer, preferably a cationic polymer.

[0060] The multi-layer container board may further include additives such as natural starch or starch derivatives, cellulose derivatives such as sodium carboxymethyl cellulose, fillers, retention and / or drainage aids, flocculation additives, deflocculating additives, dry strength additives, softeners, crosslinking aids, sizing agents, dyes and colorants, wet strength resins, solidifying agents, defoaming aids, microbial and slime control aids, or mixtures thereof.

[0061] Placing higher amounts of additives, including cellulose-based strength agents and internal sizes, in the middle plies with the UBKP, rather than distributing them throughout the multi-ply container board, can result in better glue uptake in the corrugator (due to higher starch uptake by the outer plies). Placing additives, especially internal sizes, in the middle plies rather than the outer plies also allows higher amounts of additives to be used, which can result in less mechanosorptive creep in the finished board.

[0062] It has been found to be beneficial for the multi-layer container board not to be too dry. In particular, it has been found that better fracture toughness of the multi-layer container board is obtained when dried to a particular moisture content. In some embodiments, the moisture content of the multi-layer container board is in the range of 3-17 wt%, preferably in the range of 4-14 wt%, more preferably in the range of 5-15 wt%.

[0063] In some embodiments, the multi-ply container board has a fracture toughness index GEOM (ISO / TS 17958) greater than 6 Jm / kg, preferably greater than 7 Jm / kg, and more preferably greater than 8 Jm / kg.

[0064] In some embodiments, the multi-ply container board has a SCT index GEOM (ISO 9895) greater than 23 Nm / g, preferably greater than 24 Nm / g, more preferably greater than 25 Nm / g.

[0065] In some embodiments, at least one of the outer plies of the multi-layer container board is optimized to provide a good print surface and good moisture resistance. In some embodiments, at least the outer ply intended as the outer surface of the corrugated board is optimized to provide a good print surface and good moisture resistance. In some embodiments, the optimization to provide a good print surface and good moisture resistance includes surface sizing. In some embodiments, the multi-layer container board is surface sized. In some embodiments, the multi-layer container board is surface sized with starch. In some embodiments, the multi-layer container board is surface sized with a combination of starch and at least one other functional ingredient, preferably selected from the group consisting of a crosslinking agent, a strengthening agent, and a hydrophobizing sizing agent. The crosslinking agent can be, for example, citric acid. The strengthening agent can be, for example, microfibrillated cellulose (MFC). The hydrophobizing sizing agent can be, for example, alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), SMA (styrene maleic anhydride), rosin size, or a mixture thereof.

[0066] Corrugated board contains at least one layer of uncorrugated liner and at least one layer of fluting. In typical corrugated board production, the flutes are corrugated and then glued to a linerboard. For example, corrugated board may consist of a layer of fluting sandwiched between two layers of liner.

[0067] According to a second aspect shown herein, there is provided a corrugated board comprising a multi-ply container board as defined with reference to the first aspect as fluting and / or liner.

[0068] The container board is preferably used as fluting in corrugated board. Due to the high content of NSSC pulp, the container board of the present disclosure is particularly useful as a medium for fluting used in corrugated board. Thus, in a preferred embodiment, the container board is fluted. Any type of linerboard can be used.

[0069] Although this was mentioned above, the container paperboard of the present disclosure may also be used as a liner in corrugated board for applications where a higher content of NSSC pulp can be tolerated.

[0070] According to a third aspect of the present disclosure, there is provided a method for producing a multi-ply container board for use in corrugated board, 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 said second web layer to obtain an intermediate ply on the first outer ply; c) forming a third web layer from a third pulp suspension and dewatering said third web layer to obtain a second outer ply on the intermediate ply; Includes; the first and third pulp suspensions comprising at least 70 wt. % neutral sulfite semi-chemical (NSSC) pulp on a dry weight basis; the second pulp suspension comprising at least 30 wt. % unbleached kraft pulp on a dry weight basis and at least one cellulose-based strength agent; A method is provided wherein the second pulp suspension comprises less than 30 wt% NSSC pulp on a dry weight basis.

[0071] The terms first, second and third web layer do not necessarily refer to the order in which the web layers are formed. The web layers can be formed simultaneously or individually in any order.

[0072] In some embodiments, the first, second and third web layers are formed separately using different headboxes and one or more wires, partially dewatered and then wet laminated.

[0073] In some embodiments, the first, second and third web layers are formed and partially dewatered together using a multi-headbox and single wire. For example, the third web layer may be formed and partially dewatered separately and then laminated wet with an intermediate ply to obtain a second outer ply on the intermediate ply. Alternatively, the third web layer may be formed and dewatered together with the second web layer.

[0074] In some embodiments, the first and third pulp suspensions comprise at least 80 wt% NSSC pulp on a dry weight basis, preferably at least 90 wt% NSSC pulp.

[0075] In some embodiments, the compositions of the first and third pulp suspensions are the same. In some embodiments, the composition and basis weight of the first and third outer plies are the same or nearly the same.

[0076] In some embodiments, the multi-ply container board comprises at least 50 wt% NSSC pulp on a dry weight basis, preferably at least 60 wt%.

[0077] In some embodiments, the second pulp suspension comprises at least 40 wt.%, preferably at least 50 wt.%, more preferably at least 60 wt.% unbleached kraft pulp on a dry weight basis.

[0078] In some embodiments, the unbleached kraft pulp is refined to a Schopper Riegler number in the range of 25 to 55, preferably in the range of 28 to 38, as determined according to ISO 5267-1.

[0079] In some embodiments, the cellulose-based strength agent is selected from the group consisting of cellulose fines, highly refined cellulose having a Schopper Riegler number in the range of 70 to 90, microfibrillated cellulose (MFC), and mixtures thereof.

[0080] In some embodiments, the amount of cellulose-based strength agent in the second pulp suspension is in the range of 0.5 to 100 kg / tn, preferably in the range of 1 to 25 kg / tn, based on the dry weight of the second pulp suspension.

[0081] In some embodiments, the second pulp suspension further comprises an internal sizing agent, preferably selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), rosin size, and mixtures thereof.

[0082] In some embodiments, the internal size content is higher in the second pulp suspension ply than in the first and third pulp suspension ply, preferably at least two times higher.

[0083] In some embodiments, the second pulp suspension further comprises a polymer-based solidifying agent, preferably a cationic polymer.

[0084] The process includes forming and dewatering a bulk web from a pulp suspension. Methods of forming and dewatering webs having multiple layers are well known in the art. The container board can be made on a paper or board machine adapted to produce multi-ply container board. Paper or board machines that make multi-ply container board are well known in the art. Typically, the machine layout includes a stock processing section, a wet end section, a press and drying section, and a calender and / or coating section.

[0085] The web is generally formed and dewatered in a wet end section containing one or more wires as is conventional in the art. The plies may be formed separately and laminated wet using different headboxes or may be formed together in a multi-layer headbox. The web is typically formed in a gap former, but may also be formed in a Fourdrinier type former. If formed separately, the wet plies are typically laminated or couched together before the press and dryer section of the paper machine. Prior to lamination, a reinforcing or adhesive may be applied between the middle ply and one or both of the outer plies. Preferably, the reinforcing or adhesive comprises cooked or gelatinized or uncooked starch, or a mixture of cooked or gelatinized or uncooked starch and microfibrillated cellulose (MFC). A preferred reinforcing or adhesive is cooked native starch or cooked native starch mixed with microfibrillated cellulose. In some embodiments, the reinforcing or adhesive further comprises a cross-linking agent. The cross-linking agent may be, for example, citric acid. In some embodiments, the reinforcing or adhesive further comprises an insolubilizer. The insolubilizer may be for example an amino resin, glyoxal or zirconium salt insolubilizer. The reinforcing or adhesive is preferably applied as a paste or as an aqueous dispersion using a non-contact deposition technique such as spray or foam or curtain application. Preferably, the solids content of the aqueous dispersion is in the range of 0.5-50 wt%, more preferably in the range of 1-30 wt%. The amount of reinforcing or adhesive applied is preferably 0.1-5 g / m 2 More preferably, it is in the range of 0.5 to 3 g / m2 The range is.

[0086] The web is typically subjected to further dewatering, for example the formed multi-layer web may be passed through a press section of a paper machine where the web passes between large rolls under high pressure to squeeze out as much water as possible. The press section may consist of traditional nip press units and press woven felts and / or further one or several shoe presses or extended dewatering nips. These may operate at various nip or press loads including various positions, temperatures and delay times. The press section may comprise one or more shoe presses to maximize production. If one or several shoe presses are used, these may operate at press levels of more than 800 kN / m, such as more than 1000 kN / m, such as more than 1200 kN / m, or such as more than 1450 kN / m. The removed water is typically received by a fabric or felt.

[0087] After the press section, the multi-layer web may be subjected to drying in a drying section. Drying may include, for example, drying the multi-layer web by passing the multi-layer web around a series of heated drying cylinders. Drying may typically remove the moisture content to a level of about 1-15 wt%, preferably about 2-10 wt%. It has been found that the multi-layer container board benefits from not being over-dried. In particular, it has been found that better fracture toughness of the multi-layer container board is obtained when dried to a particular moisture content. In some embodiments, the moisture content of the multi-layer container board is in the range of 3-17 wt%, preferably in the range of 4-14 wt%, more preferably in the range of 5-15 wt%.

[0088] The web may be further conditioned with heat and steam and fed between large corrugated rolls to give the finished fluting its corrugated shape.

[0089] Unless otherwise specified, the physical properties discussed in this disclosure are determined according to the following standards: Whiteness C / 2°+UV ISO 2470-1 L* C / 2°+UV ISO 5631-1 a* C / 2°+UV ISO 5631-1 b* C / 2°+UV ISO 5631-1 Basis weight ISO 536 Thickness, single sheet ISO 534 Umbrella, single sheet ISO 534 Air permeability GH ISO 5636-5 Cobb 30s ISO 535 Moisture content 50%rh ISO 287 Scott-Bond TAPPI T569 Tensile strength ISO 1924-3 Tensile Index ISO 1924-3 Tensile strength md / cd ISO 1924-3 Elongation ISO 1924-3 Tensile stiffness ISO 1924-3 Tensile stiffness index ISO 1924-3 E-Module ISO 1924-3 TEA ISO 1924-3 TEA index ISO 1924-3 TEA index ISO 1924-3 Fracture toughness ISO / TS 17958 Fracture toughness index ISO / TS 17958 Tear resistance ISO 1974 Tear index ISO 1974 SCT ISO 9895 SCT index ISO 9895 RCT ISO 12192 RCT index ISO 12192 Rupture Index ISO 2759 Bursting strength ISO 2759

[0090] Unless otherwise specified, standard methods may be applied to determine the physical and mechanical properties in both the cross direction (cd) and machine direction (md).

[0091] While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is not intended that the invention be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but the invention is intended to include all embodiments falling within the scope of the appended claims.

Claims

1. 1. A multi-layer container board for use in corrugated board, the multi-layer container board comprising: A first outer ply; a second outer ply; and at least one intermediate ply sandwiched between the first and second outer plies; Including, the first and second outer plies comprise at least 70 wt. % neutral sulfite semi-chemical (NSSC) pulp on a dry weight basis; the intermediate ply comprises at least 30 wt. % unbleached kraft pulp on a dry weight basis and at least one cellulose-based strength agent; The multi-ply container board, wherein the intermediate ply comprises less than 30 wt. % NSSC pulp on a dry weight basis.

2. 2. The multi-ply container paperboard of claim 1, wherein the first and second outer plies comprise at least 80 wt.%, preferably at least 90 wt.%, NSSC pulp on a dry weight basis.

3. 3. The multi-ply container board of claim 1 or 2, wherein the first and second outer plies are formed from pulp suspensions having the same composition.

4. 3. The multi-ply container board of claim 1 or 2, wherein the intermediate ply comprises at least 40 wt. %, preferably at least 50 wt. %, more preferably at least 60 wt. % unbleached kraft pulp on a dry weight basis.

5. 3. The multi-layer container board according to claim 1 or 2, wherein the multi-layer container board comprises at least 50 wt% NSSC pulp, preferably at least 60 wt% on a dry weight basis.

6. The multi-ply container paperboard of claim 1 or 2, wherein the unbleached kraft pulp is refined to a Schopper Riegler number, determined according to ISO 5267-1, in the range of 25 to 55, preferably in the range of 28 to 38.

7. 3. The multi-layer container paperboard of claim 1 or 2, wherein the cellulose-based strength agent is selected from the group consisting of cellulose fines, highly refined cellulose having a Schopper-Riegler number in the range of 70 to 90, microfibrillated cellulose (MFC), and mixtures thereof.

8. 3. The multi-ply container board according to claim 1 or 2, wherein the amount of cellulose based strength agent in the intermediate ply is in the range of 0.5 to 100 kg / tn, preferably in the range of 1 to 25 kg / tn, based on the dry weight of the intermediate ply.

9. 3. The multi-ply container paperboard of claim 1 or 2, wherein the intermediate ply further comprises an internal size, preferably selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), rosin size, and mixtures thereof.

10. 3. The multi-ply container board of claim 1 or 2, wherein the internal size content is higher in the middle ply than in the first and second outer plies, preferably at least twice as high.

11. 3. The multi-ply container board of claim 1 or 2, wherein the intermediate ply further comprises a polymer-based solidifying agent, preferably a cationic polymer.

12. 3. A corrugated board comprising the multi-layer container board of claim 1 or 2 as fluting and / or a liner.

13. 13. The corrugated board of claim 12 comprising multiple layers of container board as fluting.

14. 1. A method for making a multi-ply container board for use in corrugated packaging comprising the steps of: a) forming a first web layer from a first pulp suspension and dewatering said first web layer to obtain a first outer ply; b) forming a second web layer from a second pulp suspension and dewatering said second web layer to obtain an intermediate ply on the first outer ply; c) forming a third web layer from a third pulp suspension and dewatering said third web layer to obtain a second outer ply on the intermediate ply; Including, the first and third pulp suspensions comprising at least 70 wt. % neutral sulfite semi-chemical (NSSC) pulp on a dry weight basis; the second pulp suspension comprising at least 30 wt. % unbleached kraft pulp on a dry weight basis and at least one cellulose-based strength agent; The method, wherein the second pulp suspension comprises less than 30 wt.% NSSC pulp on a dry weight basis.