JP2024060684A - Cardboard - Google Patents

Abstract

To provide a cardboard having equal or better physical properties relative to conventional ones, even though a high ratio of broad-leaf tree kraft pulp is blended therein.SOLUTION: A cardboard comprising a surface layer, one or more intermediate layers, and a back-layer, includes at least 20 mass% of broad-leaf tree kraft pulp relative to the mass of the whole pulp, with the surface layer including 40 to 100 mass% of needle-leaf tree kraft pulp.SELECTED DRAWING: None

Description

The present invention relates to paperboard, particularly paperboard suitable for use as a liner for corrugated cardboard, in box making, and in laminated paper.

Corrugated cardboard is manufactured by attaching a liner to one or both sides of a corrugated core using a corrugator.
Corrugated cardboard is used for packaging various items because it is lightweight and inexpensive. The packaged items may collide with the cardboard from the inside during transportation, or may be subjected to external impact during storage. Therefore, depending on the application, the cardboard liner may be required to have high bursting strength so that it will not break even if it is subjected to impact. Generally, softwood kraft pulp is blended as the raw pulp for paperboard that requires bursting strength.

For example, Patent Document 1 proposes a cardboard liner having excellent impact resistance, which has at least a surface layer, a subsurface layer, and a back layer, in which the surface layer contains softwood kraft pulp, the content of the softwood kraft pulp in the surface layer being 80% by mass or more, the subsurface layer contains softwood kraft pulp and recycled corrugated paper pulp, the content of the softwood kraft pulp in the subsurface layer being 25% by mass or more and 35% by mass or less, the back layer contains recycled corrugated paper pulp and recycled magazine paper pulp, the content of the recycled corrugated paper pulp in the back layer being 30% by mass or more and 70% by mass or less, and the content of the recycled magazine paper pulp in the back layer being 30% by mass or more and 70% by mass or less.
Thus, softwood kraft pulp is generally used in paperboard because softwood kraft pulp has longer fibers than hardwood kraft pulp, and therefore produces paper with superior strength.

JP 2020-193396 A

The objective of the present invention is to provide a paperboard that contains a high content of hardwood kraft pulp but has physical properties equal to or better than those of conventional products.

The means for solving the above problems are as follows.
1. It has a surface layer, one or more middle layers, and a back layer;
The hardwood kraft pulp content is 20% by mass or more based on the total pulp mass,
The paperboard, wherein the surface layer contains 40 to 100% by mass of softwood kraft pulp.
2. The paperboard according to 1., wherein the back layer contains 40 to 100% by weight of hardwood kraft pulp.
3. The paperboard according to 1. or 2., wherein the back layer contains an internal paper strength agent in an amount of 0.3 to 0.8% by mass based on the mass of the pulp.
4. The paperboard according to 1. or 2., characterized in that the burst strength index is 4.5 kPa·m ² /g or more.
5. The hardwood kraft pulp content is 20 to 95 mass%, the softwood kraft pulp content is 5 to 50 mass%, and the recycled paper pulp content is 0 to 35 mass%, based on the total pulp mass;
3. The paperboard according to 1. or 2., wherein the surface layer does not contain recycled paper pulp.
6. The paperboard according to 1. or 2., characterized in that the ratio of the basis weight of the back layer to the total basis weight is 30% or more and 50% or less.
7. A liner for corrugated board made of the paperboard described in 1. or 2.

The paperboard of the present invention has physical properties equal to or greater than those of conventional paperboard. The paperboard of the present invention can be suitably used for cardboard liners, corrugating medium base paper, paperboard for paper containers, paperboard for box making, laminated paper, etc.

The paperboard of the present invention has a surface layer, one or more middle layers, and a back layer,
The hardwood kraft pulp content is 20% by mass or more based on the total pulp mass,
The surface layer contains 40 to 100% by mass of softwood kraft pulp.
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.

The paperboard of the present invention is composed of a surface layer, one or more middle layers, and a back layer, with the surface layer and back layer being the outermost layers. The number of layers of the paperboard of the present invention is three or more, and the number of middle layers can be appropriately set depending on the required basis weight, etc.

The paperboard of the present invention contains at least 20% by mass of hardwood kraft pulp relative to the total pulp mass. In addition, at least the surface layer of the paperboard of the present invention contains 40 to 100% by mass of softwood kraft pulp. As hardwood kraft pulp and softwood kraft pulp, either unbleached kraft pulp (LUKP/NBKP) or bleached kraft pulp (LBKP/NBKP) can be used, but when used in applications where whiteness is not required, unbleached kraft pulp is preferred because it has excellent strength and can be produced with low energy because there is no bleaching process.

In the paperboard of the present invention, the amount of hardwood kraft 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 65% by mass or more, and even more preferably 70% by mass or more. The amount of hardwood kraft pulp relative to the total pulp mass is preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 85% by mass or less.
In the paperboard of the present invention, the content of softwood kraft pulp relative to the total pulp mass is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more. The content of softwood kraft pulp relative to the total pulp mass is preferably 50% by mass or less, and more preferably 40% by mass or less.

The paperboard of the present invention may contain various pulps derived from wood fibers, such as hardwood kraft pulp and softwood kraft pulp, waste paper pulp, groundwood pulp (GP), refiner ground pulp (RGP), chemical pulp (CP), thermomechanical pulp (TMP), chemi-thermomechanical pulp (CTMP), and non-wood pulps obtained from kenaf, bagasse, bamboo, hemp, straw, and the like. As the other pulp, it is preferable to use inexpensive waste paper pulp. However, it is preferable that at least one of the front layer and the back layer of the paperboard of the present invention does not contain waste paper pulp, it is more preferable that the front layer does not contain waste paper pulp, and it is even more preferable that both the front layer and the back layer do not contain waste paper pulp.
The freeness of the pulp used in each layer of the present invention is not particularly limited, but is preferably 300 ml CSF or more and 600 ml or less.

As waste paper pulp, one or more of the following can be used in combination: waste paper pulp made by disintegrating unprinted waste paper such as corrugated cardboard, white, extra white, medium white, and white damaged paper; waste paper printed on fine paper, fine coated paper, medium quality paper, medium quality coated paper, and recycled paper; and pulp (DIP) made by disintegrating and deinking paper such as written paper, papers such as discarded confidential documents, magazines, and newspapers. Of these, it is preferable to use waste paper pulp derived from corrugated cardboard (hereinafter referred to as corrugated cardboard pulp). Corrugated cardboard mainly uses relatively long fiber pulp such as unbleached softwood kraft pulp, and the liner and medium are bonded together with a starch-based adhesive. The amount of printing is also small, so corrugated cardboard pulp tends to have longer fiber length, higher lignin content and starch content, and lower ink content compared to other waste paper pulps. In addition, recycled cardboard has an established recycling system and has a high recovery rate on par with recycled newspapers and magazines, so recycled cardboard pulp is inexpensive. Therefore, by using recycled cardboard pulp, it is possible to achieve both strength development in paperboard and reduced production costs. The blending ratio of recycled cardboard pulp to recycled paper pulp is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and most preferably 100% by mass.

In the paperboard of the present invention, 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 specific bursting strength, the amount of recycled paper pulp relative to the total pulp mass is preferably 45% by mass or less, more preferably 40% by mass or less, even more preferably 30% by mass or less, even more preferably 20% by mass or less, even more preferably 10% by mass or less, even more preferably 5% by mass or less, and most preferably none (0% by mass). On the other hand, recycled paper pulp is inexpensive, so the higher the amount, the lower the cost. Therefore, from the viewpoint of cost, the amount of recycled paper pulp relative to the total pulp mass is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more.

The surface layer of the paperboard of the present invention contains 40 to 100% by mass of softwood kraft pulp. This provides the paperboard of the present invention with excellent printability and bursting strength. The surface layer may contain hardwood kraft pulp, recycled paper pulp, etc., but it is preferable that the proportion of softwood kraft pulp is the highest. It is also preferable that the surface layer does not contain recycled paper pulp, that is, the papermaking fibers of the surface layer are only softwood kraft pulp and hardwood kraft pulp. The amount of softwood kraft pulp in the surface layer is preferably 60% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, even more preferably 95% by mass or more, and most preferably 100% by mass.

The back layer of the paperboard of the present invention preferably contains 40 to 100% by mass of hardwood kraft pulp. The back layer may contain softwood kraft pulp, recycled paper pulp, etc., but it is preferable that it does not contain recycled paper pulp, i.e., the papermaking fibers of the back layer are only hardwood kraft pulp and softwood kraft pulp. The amount of hardwood kraft pulp in the back layer is 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 90% by mass or more, and most preferably 100% by mass.

The paperboard of the present invention contains at least 20% by mass of hardwood kraft pulp relative to the total pulp mass contained in the paperboard, and the surface layer contains 40 to 100% by mass of softwood kraft pulp, and as long as this is satisfied, there are no particular limitations on the pulp that constitutes the middle layer. The middle layer may contain one or more types of pulps derived from wood fibers, such as softwood kraft pulp, hardwood kraft pulp, waste paper pulp, groundwood pulp (GP), refiner ground pulp (RGP), chemical pulp (CP), thermomechanical pulp (TMP), and chemi-thermomechanical pulp (CTMP), as well as non-wood pulps obtained from kenaf, bagasse, bamboo, hemp, straw, etc. When there are two or more middle layers, the pulp compositions that constitute each middle layer may be the same or different.

In the paperboard of the present invention, it is preferable from the viewpoint of burst strength that the back layer contains 0.3 to 0.8% by mass of an internal strength enhancer (hereinafter also referred to as internal strength agent) relative to the mass of pulp. As the internal strength agent to be blended in the back layer, polyacrylamide (PAM) and modified starch and other agents used in the papermaking field can be used without any particular restrictions, but it is preferable to use polyacrylamide because of its high paper strength enhancing effect. As the polyacrylamide, any of anionic, cationic and amphoteric polyacrylamide can be used. From the viewpoint of burst strength, the blending 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. The internal strength agent can be blended in an amount of more than 0.8% by mass relative to the mass of pulp, but even if it is blended in an amount of more than 0.8% by mass, the paper strength enhancing effect is almost saturated and hardly improved, and the cost becomes high.
The paperboard of the present invention may contain an internal strength agent in layers other than the back layer. In this case, the amount of the internal strength agent contained in the layers other than the back layer is not particularly limited, but is about 0.01% by mass or more and 0.5% by mass or less. The amount of the internal strength agent in each paper layer of the paperboard of the present invention may be the same or different. However, it is preferable that the amount of the internal strength agent relative to the pulp in each layer is the highest in the back layer.

The paperboard of the present invention may contain a sizing agent 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), cationic polymer-based sizing agents, and the like. One or more of these may be used. In addition, to the extent that the quality is not affected, internal additive chemicals such as aluminum sulfate, aluminum chloride, sodium aluminate, basic aluminum compounds, water-soluble aluminum compounds, polyvalent metal compounds, and silica sol may be added. The mixing ratio of the sizing agent and other internal additive chemicals in each paper layer of the paperboard of the present invention may be the same or different.

Furthermore, known fillers can be added to each layer of the paperboard of the present invention as necessary. 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, zinc hydroxide, and organic fillers such as urea-formaldehyde resin, polystyrene resin, and phenolic resin, and one or more types can be used. The filler content in each paper layer of the paperboard of the present invention may be the same or different.

The paperboard of the present invention may have a coating layer on its surface for the purpose of improving the strength of the paperboard, imparting printability, etc. Depending on the purpose, the coating layer may contain one or more of known surface treatment chemicals such as an external paper strength agent (hereinafter also referred to as a surface strength agent), a surface sizing agent, a water repellent, a pigment, a binder resin, an anti-slip agent, an antiseptic, an antifoaming agent, a viscosity modifier, and an anti-crease agent.
From the viewpoint of improving strength, the paperboard of the present invention preferably has a coating layer containing a surface strength agent. The surface strength agent may be a polyacrylamide-based agent, a polyvinyl alcohol-based agent, a starch-based agent such as oxidized starch, cationic starch, amphoteric starch, or a cellulose-based agent such as carboxymethyl cellulose, but a polyacrylamide-based agent is preferred. The coating amount of the surface strength agent is preferably 0.05 g/ ^m2 or more, and more preferably 0.1 g/ ^m2 or more.
For the purpose of improving weather resistance, the coating layer preferably contains a surface sizing agent and a water repellent agent. Examples of the surface sizing agent include those similar to those for internal addition described above, and examples of the water repellent agent include fluorine-based resins, polyamide-based resins, wax emulsions, etc.

The paperboard of the present invention preferably has a burst strength ratio (burst strength divided by basis weight) of 4.5 kPa·m ² /g or more as specified in JIS P8131:2009. Paperboards with a burst strength ratio of 4.5 kPa·m ² /g or more have excellent burst strength and are therefore less likely to break even when subjected to impact. The burst strength ratio is more preferably 4.8 kPa·m ² /g or more, even more preferably 5.0 kPa·m ² /g or more, even more preferably 5.2 kPa·m ² /g or more, and most preferably 5.5 kPa·m ² /g or more. The upper limit of the burst strength ratio of the paperboard of the present invention is not particularly limited, but is, for example, 6.5 kPa·m ² /g or less.

The basis weight of the paperboard of the present invention is not particularly limited, but can be, for example, 50 g/m ² or more and 800 g/m ² or less. The paperboard of the present invention can be suitably used, for example, as cardboard, particularly as a liner for cardboard for packaging, corrugating medium, paperboard for paper containers, paperboard for box making, paperboard for laminated paper, etc. In the case of a multi-layer paper having two or more paper layers, the total weight of all layers can be appropriately set in the range of 50 g/m ² or more and 800 g/m ² or less, and for example, when used as a cardboard liner, it can be 70 g/m ² or more and 550 g/m ² or less.

The ratio of the basis weight of the back layer to the entire basis weight is preferably from 30% by weight to 50% by weight, more preferably from 32% by weight to 45% by weight, from the viewpoint of burst strength.
The ratio of the surface layer basis weight to the entire basis weight is preferably from 10% by mass to 30% by mass, more preferably from 12% by mass to 25% by mass, from the viewpoint of burst strength.
The ratio of the basis weight of all the intermediate layers to the whole basis weight is preferably 30% by mass or more and 50% by mass or less, more preferably 35% by mass or more and 45% by mass or less, from the viewpoint of burst strength. When the intermediate layer has two or more layers, the basis weight ratio of each intermediate layer may be the same or different.

The paperboard according to the present invention has three or more paper layers, and a known manufacturing (papermaking) method and papermaking machine can be selected using a combination papermaking machine such as a Fourdrinier papermaking machine, a twin-wire papermaking machine, a cylinder papermaking machine, a gap former, and a hybrid former (on-top former). In addition, 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.

In the paperboard of the present invention, the blending of raw pulp and other ingredients differs for each layer, so that the paper stock is prepared according to each layer before papermaking.
When making the paperboard of the present invention, it is preferable to use a shaking device in the wire part. The shaking device is a device that slides the breast roll of the wire part in a direction perpendicular to the flow direction of the paper stock (also called the machine width direction). By using the shaking device, the basis weight is made uniform (improved formation), and the number of thin parts compared to other parts that are the starting point of breakage is reduced, thereby improving the bursting strength.

Furthermore, the strength of the paperboard can be improved by adjusting the papermaking conditions of each layer. For example, by setting the J/W ratio (=J/W x 100), which is the ratio of the speed (J) of the jet liquid (paper material) ejected from the headbox to the speed (W) of the wire, to less than 100%, the jet liquid is pulled by the wire, so that the pulp can be oriented in the papermaking direction (flow direction). Paperboard in which the pulp is oriented in the longitudinal direction (longitudinal direction) tends to have improved burst strength. The J/W ratio is preferably 81% or more and 99% or less, and more preferably 84% or more and 98% or less. In addition, the strength can be improved by changing the basis weight distribution of each layer to increase the basis weight of the surface layer and/or back layer, adjusting the paper strength agent content of each layer to make the paper strength agent content of the surface layer and/or back layer higher than that of the middle layer, etc.

There are no particular limitations on the manufacturing (papermaking) method and papermaking machine used after the wire part, so long as they do not impair the required strength of the paperboard and production efficiency, 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, for example, a straight-through press, a reverse press, an inbar press, a twin bar press, a pickup press, a unipress, a tri-nip press, a tri-vent press, a shoe press, a 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 a longer 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.

There are no particular limitations on the type of dryer (drying device) used in the dry part process of the papermaking machine, so long as it does not impair production efficiency or the strength of the paperboard. Any known drying device, such as the hot air drying method or multi-cylinder drying method used in normal papermaking machines, can be used. The number of drying devices installed may be one, or two or more pre-dryers, after-dryers, etc. Also, there are no particular limitations on the drying conditions, and they can be set appropriately within the normal operating range.

When a coating layer is provided, a coating liquid containing surface treatment chemicals such as a surface strength agent, a surface sizing agent, and a water repellent is applied after papermaking. There are no particular limitations on the method for applying the coating liquid, and known devices such as a two-roll size press, a gate roll coater, a shim sizer, and a spray can be used as appropriate. The calendar may be bypassed, or processing may be performed within the normal operating range.

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. The results are shown in Table 1.
(grammage)
The measurement was carried out with reference to JIS P 8124.
(density)
The thickness was calculated from the basis weight with reference to JIS P 8118 and JIS P 8124.

(Specific burst strength)
The bursting strength was measured with reference to JIS P 8131, and the bursting strength ratio was calculated by dividing the bursting strength by the basis weight.
<Macbeth concentration>
A magenta solid print (size: 2 cm length x 3 cm width) was printed on the surface layer of the prepared paperboard using a flexographic printer (printability tester F1, manufactured by IGT Testing Systems Co., Ltd.). After one day, the Macbeth density after printing was measured using a Macbeth densitometer (Gretag Macbeth RD-19).
Under these conditions, the Macbeth density after printing is preferably 1.1 or more.
<Glossiness>
In accordance with JIS Z8741, the gloss (light incidence angle 75 degrees) of the magenta solid portion of the paperboard surface layer after the flexographic printing was measured using a gloss meter (True GLOSS GM-26PRO, manufactured by Murakami Color Research Laboratory).
Under these conditions, the preferred gloss level after printing is 12% or more.

Examples and Comparative Examples The pulp used for each layer is shown in Table 1. For the mechanical treatment of the pulp, a double conifer was used for the surface layer, and a double disc refiner was used for the middle and back layers. The freeness (CSF) at the outlet of the double conifer and double disc refiner is shown in Table 1 as the freeness (CSF) immediately after mechanical treatment.
In each layer, an internal paper strength agent (PAM) and an internal sizing agent (rosin-based) were added in the amounts shown in Table 1 relative to the pulp to prepare paper stock.
The paper stock for each layer was combined using a multi-layer paperboard machine in the following order: back layer (35% by mass), two middle layers (25% by mass in the back, 20% by mass in the front), and front layer (20% by mass) so that the total weight of all layers was 230 g/ ^m2 . The J/W ratio of the front layer, middle layer (2 layers), and back layer was all 90%, and a shaking device was operated in the wire part when the back layer was made, to obtain paperboard.

The paperboards obtained in Examples 1 to 9 of the present invention had strength and printability equal to or greater than those of the paperboards obtained in Comparative Examples 1, 2, 4, and 5, which contained large amounts of softwood kraft pulp (NUKP) or recycled paper pulp.
From Examples 1 and 2 and Examples 8 and 9, when the internal strength enhancer was contained in a large amount, the paperboard containing a large amount of hardwood kraft pulp (LUKP) had a higher burst strength than the paperboard containing a large amount of softwood kraft pulp (NUKP). Also, the paperboard of Example 3, in which the back layer was 100% LUKP, had a higher burst strength than the paperboard of Example 4, in which the back layer was 80% LUKP.
The paperboard of Comparative Example 3 had equal or greater strength, but had a low gloss (75°) value after printing and was inferior in printability.
Comparative Example 4 is a general paperboard containing a high amount of recycled paper pulp, and Comparative Example 5 is a paperboard in which the amount of internal strength agent added was increased to improve bursting strength. The paperboards of Comparative Examples 4 and 5, which contain a large amount of recycled paper pulp, were inferior in bursting strength, and even when the amount of internal strength agent was increased, they did not reach the paperboard of the present invention.

Claims (7)

It has a surface layer, one or more middle layers, and a back layer,
The hardwood kraft pulp content is 20% by mass or more based on the total pulp mass,
The surface layer of the paperboard contains 40 to 100% by mass of softwood kraft pulp. The paperboard according to claim 1, characterized in that the back layer contains 40 to 100% by mass of hardwood kraft pulp. The paperboard according to claim 1 or 2, characterized in that the back layer contains 0.3 to 0.8% by mass of an internal paper strength enhancer relative to the mass of pulp. 3. The paperboard of claim 1 or 2, having a burst strength index of 4.5 kPa·m ² /g or more. With respect to the total pulp mass, the hardwood kraft pulp content is 20 to 95 mass%, the softwood kraft pulp content is 5 to 50 mass%, and the recycled paper pulp content is 0 to 35 mass%,
3. The paperboard according to claim 1 or 2, wherein the surface layer does not contain recycled paper pulp. The paperboard according to claim 1 or 2, characterized in that the ratio of the basis weight of the back layer to the total basis weight is 30% or more and 50% or less. A cardboard liner made of the paperboard according to claim 1 or 2.