Paperboard

Abstract

To provide a paperboard with excellent flexographic printing quality.SOLUTION: A paperboard includes a coating layer on at least one surface of a paper substrate, wherein the coating layer contains an epichlorohydrin compound.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to paperboard, particularly paperboard that is printed, such as corrugated cardboard liners and boxboard. [Background technology]

[0002] Corrugated cardboard is manufactured by laminating a liner to one or both sides of a corrugated core using a corrugator. Corrugated cardboard is lightweight and inexpensive, so it is used to package a variety of items. Because corrugated cardboard requires strength, it is generally made from softwood kraft pulp. In recent years, the volume of mail-order sales has increased dramatically, and some mail-order companies are using highly designed cardboard packaging with unique printing. However, conventional paperboard with a high content of softwood kraft pulp has poor cushioning and poor adhesion to the printing plate, resulting in inferior print quality.

[0003] In Patent Document 1, the present inventors have disclosed a method for manufacturing a lining sheet having at least a surface layer, a surface middle layer, a back middle layer, and a back layer, and having 4 to 6 layers, The surface layer contains 50 to 100% by mass of unbleached softwood kraft pulp, the surface middle layer, the back middle layer, and the back layer contain recycled corrugated cardboard pulp; The surface middle layer and the back middle layer contain 95% by mass or less of recycled corrugated cardboard pulp relative to the entire pulp, and the back layer contains less than 90% by mass of recycled corrugated cardboard pulp relative to the entire pulp, All layers contain an internal sizing agent, and the content of the total internal sizing agent per 100 parts by mass of the total pulp is 0.2 to 1.0 parts by mass; Total basis weight 330g / m 2 That's all, The Cobb water absorbency of the surface layer and the back layer after a measurement time of 2 minutes is 5 g / m 2 More than 50g / m 2 The absolute value of the difference in Cobb water absorbency between the surface layer and the back layer is 10 g / m or less. 2 The paperboard proposed has a high basis weight, which is less likely to break when folded as described below, and which has excellent print quality.

[0004] In Patent Document 2, a surface layer, one or more middle layers, and a back layer are provided. Contains 20% or more by mass of hardwood kraft pulp relative to the total pulp mass, The surface layer contains 50 to 100% by mass of softwood kraft pulp, Compression energy: 2.0 to 25.0 gf·cm / cm 2 We have proposed a paperboard with excellent strength and print quality, with a compression recovery rate of 40 to 95%. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 7323113 [Patent Document 2] Patent application No. 2023-038484 DISCLOSURE OF THE INVENTION [Problem to be solved by the invention]

[0006] The paperboards described in Patent Documents 1 and 2 have improved printability by modifying the pulp composition. As a result of extensive research, the present inventors have discovered a configuration that can improve printability without relying on the pulp composition, and have completed the present invention. An object of the present invention is to provide a paperboard that has excellent flexographic printing quality. [Means for solving the problem]

[0007] The means for solving the problems of the present invention are as follows. 1. A paper substrate having a coating layer on at least one side thereof; The paperboard characterized in that the coating layer contains an epichlorohydrin-based compound. 2. The paperboard according to 1., wherein the coating layer contains either a starch-based compound, a polyacrylamide-based compound, or both. 3. The paper substrate has a paper layer containing recycled corrugated cardboard pulp, 3. The paperboard according to 1. or 2., wherein the coating layer is provided on the paper layer. 4. The coating amount (solid content) of the epichlorohydrin compound per side is 0.02 g / m 2 The paperboard according to any one of 1. to 3. above, characterized in that: 5. The paperboard according to any one of 1. to 4., wherein flexographic printing is applied to the coating layer. [Effects of the Invention]

[0008] The paperboard of the present invention has excellent print quality. Because the paperboard of the present invention is provided with excellent print quality by the coating layer, print quality can be improved regardless of the pulp composition of the paper base material. DETAILED DESCRIPTION OF THE INVENTION

[0009] The paperboard of the present invention has a coating layer on at least one side of a paper substrate, The coating layer contains an epichlorohydrin-based compound. In this specification, the expression "A to B" (A and B are numerical values) means a numerical range including both ends, that is, A or more and B or less.

[0010] ·Paper base material In the present invention, the paper substrate may be either a paper consisting of only a single paper layer or a multilayer paper having two or more paper layers. When the paper substrate is a multilayer paper, it is composed of a surface layer, an optional middle layer, and a back layer, with the surface layer and back layer being the outermost layers. The number of layers in the multilayer paper can be set according to the required basis weight, etc. Note that the back layer refers to the layer located on the bottom surface during papermaking, i.e., the layer in contact with the main wire or felt, and the surface layer refers to the layer opposite the back layer, i.e., the layer located on the top surface during papermaking.

[0011] The pulp contained in the paper base material is not particularly limited, and can be one or a mixture of two or more of various pulps derived from wood fibers, such as unbleached hardwood kraft pulp (LUKP), unbleached softwood kraft pulp (NUKP), bleached hardwood kraft pulp (LBKP), bleached softwood kraft pulp (NBKP), recycled paper pulp, groundwood pulp (GP), refiner ground pulp (RGP), chemical pulp (CP), thermomechanical pulp (TMP), and chemithermomechanical pulp (CTMP), as well as non-wood pulps obtained from kenaf, bagasse, bamboo, hemp, straw, etc. The Canadian Standard Freeness of the pulp used in the present invention is not particularly limited, but is preferably 300 ml CSF or more and 600 ml CSF or less. When the paper base material is a multi-layer paper, the pulp compositions and Canadian Standard Freenesses of the respective paper layers may be the same or different.

[0012] Although recycled paper pulp is known to have poor printability, the paperboard of the present invention has a specific coating layer, allowing high-quality printing to be performed on this coating layer. Therefore, the pulp composition of the paper layer on which the coating layer of the paper substrate is formed is not particularly limited, and recycled paper pulp can be incorporated. Examples of recycled paper pulp include recycled paper pulp obtained by disintegrating unprinted recycled paper such as recycled corrugated cardboard, recycled magazine paper, white paper, extra white paper, medium white paper, and white damage; recycled paper printed on fine paper, fine coated paper, medium quality paper, medium quality coated paper, and recycled paper; and pulp obtained by disintegrating and deinking (DIP) recycled paper such as written paper, discarded confidential documents, recycled magazine paper, and recycled newspaper paper. One or more of these pulps can be used in combination. Among these, recycled paper pulp derived from recycled corrugated cardboard (hereinafter referred to as recycled corrugated cardboard pulp) and recycled magazine paper are preferred from the standpoints of cost and strength.

[0013] The amount of recycled paper pulp relative to the total pulp contained in the paper stock that constitutes the paper layer on which the coating layer of the paper base material is formed (all paper stock in the case of single-layer paper, and paper stock that constitutes at least one of the outermost paper layers in the case of multi-layer paper) is preferably 30% by mass or more, more preferably 50% by mass or more, even more preferably 60% by mass or more, even more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 85% by mass or more, and even more preferably 90% by mass or more, in order to reduce the environmental impact by promoting the use of recycled paper pulp, which is a recycled material.

[0014] When the paper base material is a multi-layer paper, the amount of recycled paper pulp relative to the total pulp mass is not particularly limited, but the lower the amount of recycled paper pulp, the higher the bursting strength can be maintained. Therefore, from the viewpoint of strength, the amount of recycled paper pulp relative to the total pulp mass is preferably 80% by mass or less, more preferably 70% by mass or less, even more preferably 50% by mass or less, even more preferably 30% by mass or less, and even more preferably 10% by mass or less. On the other hand, from the viewpoint of reducing the environmental load, the amount of recycled paper pulp relative to the total pulp mass is preferably 20% by mass or more, more preferably 40% by mass or more, even more preferably 60% by mass or more, even more preferably 80% by mass or more, even more preferably 85% by mass or more, and even more preferably 90% by mass or more.

[0015] The paper base material may contain known internal chemicals such as retention aids, bulking agents, dry strength improvers, wet strength improvers, drainage improvers, water retention agents, dyes, sizing agents, etc. When the paper base material is a multi-layer paper, the type and content of the internal chemicals may be different for each paper layer or may be the same. The paper substrate of the present invention preferably contains a paper strength agent (hereinafter also referred to as an internal paper strength agent) as an internal chemical. As the internal paper strength agent, polyacrylamide (PAM)-based agents, modified starch, and other agents used in the papermaking field can be used without any particular restrictions, but polyacrylamide-based agents are preferred because of their high paper strength enhancing effect. As the polyacrylamide-based agents, any of anionic, cationic, and amphoteric agents can be used.

[0016] When the paper base material consists of a single paper layer, it is preferable that the internal strength agent be contained in an amount of 0.3 to 0.8% by mass relative to the pulp mass from the viewpoints of burst strength and compressive strength. In terms of burst strength and compressive strength, the amount of internal strength agent added relative to the pulp mass in a paper base material consisting of a single paper layer is more preferably 0.35% by mass or more, even more preferably 0.4% by mass or more, and even more preferably 0.45% by mass or more. The internal strength agent can be added in an amount exceeding 0.8% by mass relative to the pulp mass, but even if the amount exceeds 0.8% by mass, the paper strength enhancing effect is almost saturated and hardly any improvement is achieved, resulting in high costs.

[0017] When the paper base material is a multilayer paper, it is preferable that the back layer contain 0.3 to 0.8% by mass of the internal strength agent relative to the mass of pulp from the viewpoints of burst strength and compressive strength. The amount of the internal strength agent relative to the mass of pulp in the back layer is more preferably 0.35% by mass or more, even more preferably 0.4% by mass or more, and even more preferably 0.45% by mass or more from the viewpoints of burst strength and compressive strength. The internal strength agent can be blended in at more than 0.8% by mass relative to the mass of pulp, but even if it is blended in at more than 0.8% by mass, the paper strength enhancing effect is almost saturated and there is almost no improvement, and the cost is high. When the paper base material is a multilayer paper, the layers other than the back layer may also contain the above-mentioned internal strength agent. In this case, the amount of internal strength agent contained in the layers other than the back layer is not particularly limited, but is approximately 0.01% by mass or more and 0.5% by mass or less. The type and amount of internal strength agent in each paper layer of the multilayer paper base material may be the same or different. However, it is preferable that the amount of internal strength agent relative to the pulp in each layer is the highest in the back layer.

[0018] A sizing agent can be added to the paper substrate to improve the weather resistance of the paperboard. Examples of sizing agents include rosin-based sizing agents, rosin emulsion-based sizing agents, α-carboxymethyl saturated fatty acids, neutral rosin-based sizing agents, alkyl ketene dimers (AKD), alkenyl succinic anhydrides (ASA), and cationic polymer-based sizing agents. One or more of these can be used. Furthermore, internal additives such as aluminum sulfate, aluminum chloride, sodium aluminate, basic aluminum compounds, water-soluble aluminum compounds, polyvalent metal compounds, and silica sol can be added to the extent that they do not affect the quality.

[0019] Furthermore, known fillers can be added to the paper substrate as needed. Examples of fillers include inorganic fillers such as kaolin, calcined kaolin, delaminated kaolin, clay, calcined clay, delaminated clay, illite, heavy calcium carbonate, light calcium carbonate, light calcium carbonate-silica composite, magnesium carbonate, barium carbonate, titanium dioxide, zinc oxide, silicon oxide, amorphous silica, aluminum hydroxide, calcium hydroxide, magnesium hydroxide, and zinc hydroxide, as well as organic fillers such as urea-formalin resin, polystyrene resin, and phenolic resin. One or more types can be used. When the paper substrate is a multi-layer paper, the type and content of the filler in each paper layer can be the same or different.

[0020] The basis weight of the paper substrate is not particularly limited, but is, for example, 50 g / m 2 More than 800g / m 2 In the case of multi-layer paper, the basis weight of each paper layer is 10 g / m 2 More than 250g / m 2 The paperboard of the present invention can be suitably used for applications in which printing is applied to at least one surface thereof, such as a cardboard liner, a paperboard for paper containers, a paperboard for box making, etc. For example, when used as a cardboard liner, the thickness of the paperboard can be 70 g / m or less. 2 More than 550g / m 2 It can be as follows:

[0021] Coating layer The paperboard of the present invention has a coating layer containing an epichlorohydrin compound on at least one side of the paper base, and can also have this coating layer on both sides of the paper base. When a coating layer is provided on only one side, the coating layer can be provided on either the front or back side of the paper base. The paperboard of the present invention has a coating layer containing an epichlorohydrin-based compound, and high-quality flexographic printing can be performed on this coating layer. Although epichlorohydrin-based compounds are used as wet strength agents in the papermaking industry, their ability to improve flexographic printing quality is a novel finding that has not been previously known.

[0022] Examples of epichlorohydrin compounds include polyamine epichlorohydrin resins and polyamide epichlorohydrin resins, and commercially available products can also be used, such as WS4011 (polyamine epichlorohydrin resin), WS4020, WS4030, WS4027, and TS4070 (all polyamide epichlorohydrin resins) from Seiko PMC Corporation, and Arafix AF-100, AF-251, and AF-255 (all polyamide epichlorohydrin resins) from Arakawa Chemical Industries, Ltd.

[0023] The coating amount (solid content) of the epichlorohydrin compound per side is not particularly limited as long as the effects of the present invention are achieved. However, since the printing quality tends to improve as the coating amount increases, it is preferred that the coating amount be 0.02 g / m 2 More than 0.03 g / m is preferable. 2 More preferably, 0.05 g / m or more 2 More preferably, 0.07 g / m or more 2 More preferably, 0.1 g / m 2 More preferably, 0.3 g / m 2 More preferably, 0.5 g / m 2 More preferably, 0.7 g / m 2 The upper limit of the coating amount (solid content) of the epichlorohydrin compound per side is not particularly limited, but it is preferably 3 g / m 2Even if the coating weight is increased further, the effect of improving print quality will reach a saturation point and will not improve further, and the amount of chemicals used will increase, resulting in higher costs. The upper limit of this coating weight is 2.5 g / m 2 Less than 2g / m is preferred 2 Less than 1.5 g / m is more preferable. 2 More preferably, 1.0 g / m 2 Even more preferred are the following:

[0024] The coating layer containing an epichlorohydrin-based compound preferably contains either a starch-based compound, a polyacrylamide-based compound, or both. The use of an epichlorohydrin-based compound in combination with either a starch-based compound, a polyacrylamide-based compound, or both further improves flexographic printing quality, although the mechanism is unclear. As the starch-based compound, any of those known in the papermaking field can be used without particular limitation, such as oxidized starch, cationized starch, amphoteric starch, hydroxyethyl starch (HES), phosphate ester starch, esterified starch, dextrin, etc. As the polyacrylamide-based compound, commercially available products can be used, such as Harmide from Harima Chemicals, the DS series, DH series, DA series, and DG series from Seiko PMC, and Polystron from Arakawa Chemical.

[0025] The coating amount (solid content, total coating amount in the case of both) of the starch-based compound and the polyacrylamide-based compound per side is not particularly limited as long as the effect of the present invention is exhibited. However, from the viewpoint of improving the flexographic printing quality, it is preferable that the coating amount is 0.1 g / m or less. 2 More than 0.2g / m is preferable. 2 More preferably, 0.3 g / m or more 2 More preferably, 0.4 g / m 2 More preferably, 0.5 g / m 2 The upper limit of the coating amount (solid content) per side is not particularly limited, but it is preferably 3 g / m 2Even if the coating weight is increased further, the effect of improving print quality will reach a saturation point and will not improve further, and the amount of chemicals used will increase, resulting in high costs. The upper limit of this coating weight is 2.5 g / m 2 Less than 2 g / m is preferred 2 Less than 1.5 g / m is more preferable. 2 More preferably, 1.0 g / m 2 Even more preferred are the following:

[0026] In addition to the epichlorohydrin-based compounds, starch-based compounds, and polyacrylamide-based compounds described above, the coating layer may contain one or more of known surface treatment chemicals such as external paper strength agents (hereinafter also referred to as surface strength agents), surface sizing agents, water repellents, pigments, binder resins, anti-slip agents, preservatives, antifoaming agents, viscosity modifiers, and anti-cracking agents.

[0027] The higher the smoothness, the better the printing quality. However, to increase the smoothness, it is necessary to increase the calendering line pressure during papermaking, which leads to a decrease in paper thickness and compressive strength. From the viewpoints of print quality and strength, the paperboard of the present invention preferably has a smoothness (Oken method, JIS P8155:2010) of 20 seconds or less, more preferably 15 seconds or less, and even more preferably 10 seconds or less. The lower limit of this smoothness is not particularly limited, but is, for example, 5 seconds or more.

[0028] If the Cobb absorbency is too high, the ink may sink into the paper substrate during printing, resulting in a decrease in print density.On the other hand, if it is too low, the ink may dry poorly during printing, leading to set-off.Furthermore, lowering the Cobb absorbency requires increasing the amount of sizing agent added, which also leads to increased costs. The paperboard of the present invention has a Cobb water absorption (JIS P 8140:1998, contact time 120 seconds) of 22 g / m2 or less from the viewpoint of print quality and runnability during printing. 2 More than 40g / m 2 Less than 24 g / m is preferred 2 More preferably, 26 g / m 2 More preferably, 36 g / m 2Less than 32 g / m is more preferable. 2 The following is even more preferred:

[0029] ·Manufacturing method The paper base material can be produced by a known manufacturing (papermaking) method using a known papermaking machine such as a Fourdrinier papermaking machine, twin-wire papermaking machine, cylinder papermaking machine, gap former, hybrid former (on-top former), etc., or in the case of multilayer paper, a papermaking machine that combines these. Furthermore, the pH during papermaking may be in the acidic range (acidic papermaking), pseudo-neutral range (pseudo-neutral papermaking), neutral range (neutral papermaking), or alkaline range (alkaline papermaking), and after papermaking in the acidic range, an alkaline agent may be applied to the surface of the paper layer.

[0030] There are no particular limitations on the manufacturing (papermaking) method and papermaking machine used after the wire part, and machines used in ordinary papermaking machines can be used. The type of press in the press part of the papermaking machine is not particularly limited, and any known press device can be used, such as a straight-through press, reverse press, inver press, twin bar press, pickup press, unipress, tri-nip press, tri-vent press, shoe press, mini shoe press, etc. There are also no particular limitations on the press conditions, and they can be set appropriately within the normal operating range, but since extending the press time makes it easier for the fibers to hydrogen bond and improves the strength of the paperboard, it is preferable to make the paper at a low speed within a range that does not impair production efficiency.

[0031] There are no particular limitations on the type of dryer (drying device) used in the drying part of a paper machine, and known drying devices such as hot air drying systems and multi-cylinder drying systems used in ordinary paper machines can be used. The number of drying devices installed can be one, or two or more pre-dryers, after-dryers, etc. There are also no particular limitations on the drying conditions, and they can be set appropriately within the normal operating range.

[0032] The coating layer is formed by applying a coating solution containing at least an epichlorohydrin compound to the paper after papermaking and drying the applied solution. There are no particular limitations on the method for applying the coating solution, and known devices such as a two-roll size press, a gate roll coater, a shim sizer, and a sprayer can be used as appropriate. The calender may be bypassed or may be processed within the normal operating range. [Example]

[0033] The present invention will be described below with reference to examples, but the present invention is not limited to the following examples. The paperboards obtained in the following Examples and Comparative Examples were measured by the following methods, and the results are shown in Table 1.

[0034] (Basic weight) Measurements were made based on JIS P8124:2011. (Thickness) Measurements were taken based on JIS P8118:2014. (density) It was calculated from the basis weight and thickness.

[0035] (coating amount) The coating liquid on the applicator roll was scraped off for a predetermined time and weighed to calculate the wet coating amount, and then the coating amount was calculated from the wet coating amount and the component concentrations in the coating liquid.

[0036] (Smoothness: Ohken) Measurements were made based on JIS P8155:2010. (Cobb water absorption) Measurement was carried out based on JIS P 8140:1998 with a contact time of 120 seconds.

[0037] (printability) The coated surface of the paperboard was subjected to a printability test using a printability tester (IGT F1, manufactured by IGT Testing Systems Co., Ltd., with an anilox roll of 4.5 cm 3 / m 2) was used to perform solid flexographic printing using red ink (DF037, manufactured by Sakata Inx Corporation) at a printing pressure of 10 N and a printing speed of 0.5 m / s, and the following evaluations were carried out. Print gloss The 75-degree gloss was measured using a gloss meter (VG7000 manufactured by Nippon Denshoku Industries Co., Ltd.). ·Print density The magenta density of the printed area was measured using a spectrodensitometer (Exact eXact spectrodensitometer manufactured by Sakata Inx Engineering Corporation). Whiteout rate A print sample image (3.5 x 15 cm) was scanned using a scanner (GT-X830 manufactured by EPSON Corporation), and the image was binarized using image analysis software (Image Pro manufactured by Media Cybernetics Inc.). The image was then visually compared with the image to adjust the threshold so that white areas, which were printing defects, could be identified, and their area ratio was measured.

[0038] Examples and Comparative Examples "Preparation of paper substrate" The pulp used was 100% by mass of recycled corrugated cardboard pulp (CSF 430 ml) for the back and middle layers, and 10% by mass of softwood unbleached kraft pulp (NUKP) and 90% by mass of recycled corrugated cardboard pulp (CSF 400 ml when mixed) for the surface layer. The back and middle layers were made of 100% pulp by mass, and the strength agents added were 1.0% by mass of polyacrylamide (PAM) and 1.5% by mass of aluminum sulfate for the back and middle layers. The strength agents added to the surface layer were 1.0% by mass of PAM, 1.5% by mass of aluminum sulfate, and 0.3% by mass of sizing agent. The paper was combined in a mass ratio of back layer:middle layer:surface layer = 20:65:15, with a target basis weight of approximately 170 g / m. 2 A paper substrate was prepared.

[0039] "Preparation of coating liquid" Agent A: Polyamine epichlorohydrin resin Seiko PMC Co., Ltd. WS4011 (wet strength agent) Agent B: Polyamide epichlorohydrin resin Seiko PMC Co., Ltd. WS4020 (wet strength agent) Agent C: Styrene acrylic resin Harima Chemical Co., Ltd. KN275 (anionic surface sizing agent) Agent D: Styrene acrylic resin Seiko PMC SE2202 (cationic surface sizing agent) Starch: Manufactured by Nihon Shokuhin Kako Co., Ltd., MS#3800 PAM: Manufactured by Seiko PMC Co., Ltd. ST5017

[0040] The chemicals shown in Table 1 were mixed to prepare a coating liquid. Each coating solution was applied to both sides of the paper substrate. Table 1 shows the coating amount (solid content) per side.

[0041] [Table 1]

[0042] The paperboards of the present invention obtained in Examples 1 to 12 and Examples 13 to 20 all had improved print gloss, print density, and whiteout rate compared to the paperboards obtained in Comparative Examples 1 and 2, which did not contain the epichlorohydrin-based compounds Agents A and B, respectively. Furthermore, the flexographic printing quality tended to improve as the coating weight of the epichlorohydrin-based compound increased. Examples 5 to 8, 9 to 11, and 17 to 20 confirmed that the combined use of starch or PAM improved print quality. In Comparative Examples 3 to 10, surface sizing agents C and D were applied instead of epichlorohydrin compounds. However, no improvement in flexographic printing quality was observed whether an anionic or cationic surface sizing agent was applied; in fact, the quality actually decreased in both cases.

Claims

1. A coating layer is provided on at least one surface of the paper substrate, The paperboard characterized in that the coating layer contains an epichlorohydrin-based compound.

2. 2. The paperboard of claim 1, wherein the coating layer contains a starch-based compound, a polyacrylamide-based compound, or both.

3. The paper substrate has a paper layer containing recycled corrugated cardboard pulp, 3. The paperboard of claim 1, wherein the coating layer is provided on the paper layer.

4. The coating amount (solid content) of the epichlorohydrin compound per side is 0.02 g / m ２ 3. The paperboard according to claim 1 or 2, wherein:

5. 3. The paperboard according to claim 1, wherein the coating layer is flexographically printed.

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