JP2024060762A - Paper substrate for molding, and method for manufacturing the paper substrate for molding - Google Patents

Abstract

[Problem] The present invention provides a paper substrate for molding into a mold that is excellent in processability when obtaining a molded paper body with sufficient strength for practical use, and a method for producing the same. [Solution] The paper substrate for molding into a mold of the present invention has at least a first surface layer, a middle layer, and a second surface layer, in that order. The freeness of the pulp in the first surface layer is lower than the freeness of the pulp in the middle layer. And the freeness of the pulp in the second surface layer is higher than the freeness of the pulp in the middle layer. And the aspect ratio of the tensile strength of the paper substrate for molding into a mold of the present invention is less than 2. [Selected Figure] None

Description

Application for application of Article 30, Paragraph 2 of the Patent Act submitted October 12, 2022 2022 Tokyo International Packaging Exhibition (TOKYO PACK 2022)

The present invention relates to a paper substrate for molding and a method for manufacturing the paper substrate for molding.

Traditionally, plastic containers have been used as food packaging containers such as lunch boxes and food trays, as well as packaging containers for industrial products such as CD cases and cases for electrical components. As a measure against recent environmental problems, it has been proposed to replace these plastic containers with paper molded bodies made from natural pulp (for example, Patent Document 1).

The paper molded body can have various shapes, from molded bodies with almost no drawing depth, such as shallow plates and cups, to molded bodies with a deep drawing depth. Therefore, it is required to manufacture paper molded bodies of various shapes from a paper base material.
In general, when a deep drawn body is formed, the paper base material is stretched more than when the drawn body is shallow. Also, the paper layer on the outside of the body is stretched more than the paper layer on the inside during drawing. Therefore, the paper layer on the outside of the deep drawn body is easily cut during forming.

Patent Document 1 proposes that the breaking elongation of the paper layer on the outside of the molded body be 5% or more to prevent the paper from breaking. Patent Document 1 also proposes that the overall density of the paper base material be 0.4 to 0.7 g/ ^cm3 to obtain a molded paper body with high stiffness during drawing.

JP 2002-200726 A

However, according to the inventor's investigation, the paper base material proposed in Patent Document 1 has room for improvement in processability, since cracks, tears, and wrinkles due to misalignment of the surface layer occur when a deep drawn body is formed in a mold. It is also difficult to achieve both processability during drawing and strength of the formed body. It is not realistic to set the overall density of the paper base material within the above numerical range, due to the high operational difficulty.

The present invention provides a molded paper substrate that is excellent in processability when producing a molded paper body that has sufficient strength for practical use, and a method for producing the same.

The inventors discovered that the above problem could be solved by using a short wire multi-cylinder papermaking machine and optimizing the freeness of the pulp in each layer of the multi-layer paper for molding.

The present invention has the following aspects.
[1] A molded paper substrate having at least a first surface layer, a middle layer, and a second surface layer, in that order; the freeness of the pulp of the first surface layer is lower than the freeness of the pulp of the middle layer; the freeness of the pulp of the second surface layer is higher than the freeness of the pulp of the middle layer; and the aspect ratio of the tensile strength is less than 2.
[2] The paper substrate for mold forming described in [1], wherein the freeness of the pulp of the first surface layer is lower than the freeness of the pulp of the second surface layer.
[3] A method for producing a molded paper base material having at least a first surface layer, a middle layer, and a second surface layer in this order, the method comprising: combining a first papermaking layer having at least a first surface layer obtained by making a pulp slurry for a first surface layer, which contains pulp having a lower freeness than the pulp of the middle layer, using a short wire multi-cylinder papermaking machine; and a second papermaking layer having at least a second surface layer obtained by making a pulp slurry for a second surface layer, which contains pulp having a higher freeness than the pulp of the middle layer, using a short wire multi-cylinder papermaking machine.
[4] The manufacturing method described in [3], in which the freeness of the pulp slurry for the first surface layer is lower than the freeness of the pulp slurry for the second surface layer.

The present invention provides a molded paper substrate that is easy to process and has sufficient strength for practical use, and a method for producing the same.

"Freeness" refers to disintegrated freeness (csf). Disintegrated freeness refers to the Canadian Standard Freeness value measured using a pulp slurry obtained by disintegrating a paper base material. In other words, freeness is the freeness value measured using a Canadian Standard Freeness Tester in accordance with JIS-P8121-2:2012 after disintegrating a sample using a standard disintegrator in accordance with JIS-P8220-2:2012.
The "cross direction" is defined as the direction perpendicular to the paper machine direction on the surface of the paper base. The paper machine direction can be confirmed from the orientation direction of the fibers of the paper base. The "machine direction" is the direction perpendicular to the cross direction on the surface of the paper base.
The use of "to" indicating a range of numerical values means that the numerical values before and after it are included as the lower limit and upper limit.
The lower and upper limits of the numerical ranges disclosed in this specification can be combined in any manner to create new numerical ranges.

Several embodiments are described in detail below. The following disclosure relates to a description of representative examples of embodiments, and the present invention is not limited to the following disclosure.

<Paper base material for mold molding>
The paper substrate for molding of the present invention is a multi-layered paper having at least a first surface layer, a middle layer, and a second surface layer in this order. The paper substrate for molding of the present invention may be a multi-layered paper having a three-layer structure, or may be a multi-layered paper having about four to nine layers.

The first surface layer is the layer disposed on the outermost side of the multi-layered paper substrate for molding in a mold. The second surface layer is the layer disposed on the innermost side (opposite the first surface layer) of the multi-layered paper substrate for molding in a mold. In other words, the first surface layer and the second surface layer are the outermost layers of the paper substrate for molding in a mold. Therefore, the first surface layer and the second surface layer are in direct contact with the mold during molding in a mold.
The raw material composition of each of the first and second surface layers may be the same as or different from each other, and the raw material composition of each of the first and second surface layers may be the same as or different from that of the middle layer.

The middle layer is a layer disposed between the first surface layer and the second surface layer. The middle layer may be a single layer, or may have a multi-layer structure of about 2 to 7 layers. When the middle layer has a multi-layer structure, the raw material composition of each layer of the middle layer may be the same or different.

The mold-molding paper substrate may have a first undersurface layer between the first surface layer and the middle layer, which is in contact with the first surface layer, or may not have the first undersurface layer. If the middle layer has a multi-layer structure of three or more layers, the layer immediately below the first surface layer is the first undersurface layer. The first undersurface layer can also be said to be the layer that is in contact with the first surface layer on the inside of the mold-molding paper substrate.

The mold-molding paper substrate may have a second undersurface layer between the second surface layer and the middle layer, which is in contact with the second surface layer, or may not have the second undersurface layer. If the middle layer has a multi-layer structure of three or more layers, the layer immediately below the second surface layer is the second undersurface layer. The second undersurface layer can also be said to be the layer that is in contact with the second surface layer on the inside of the mold-molding paper substrate.

Each layer of the mold forming paper base material is made mainly of cellulose pulp. The main component of each layer means a component that accounts for 50% by mass or more of the components constituting each layer.
The type of cellulose pulp is not particularly limited. From the viewpoint of strength, chemical pulp is preferred. The chemical pulp is not particularly limited. For example, hardwood kraft pulp (LKP) and softwood kraft pulp (NKP) are preferred. At least a part of the cellulose pulp in each layer may be the same type, or may be different from each other.
It is particularly preferred that all layers of the moldable paper substrate contain LKP as the cellulose pulp, since the improved flexibility further improves processability during molding.

The cellulose pulp may be bleached or unbleached. Hereinafter, unless otherwise specified, LKP and NKP are terms that include bleached or unbleached pulp, respectively. Hardwood bleached kraft pulp may be referred to as "LBKP." Softwood bleached kraft pulp may be referred to as "NBKP."
Examples of LKP include acacia wood and eucalyptus wood, and examples of NKP include radiata pine wood.
The cellulose pulp may be used alone or in combination of two or more kinds.

Each layer of the molded paper base material may contain pulps other than NKP and LKP. Examples of other pulps include mechanical pulps such as stone ground pulp (SGP), pressurized stone ground pulp (PGW), refiner ground pulp (RGP), thermo ground pulp (TGP), chemi-ground pulp (CGP), groundwood pulp (GP), and thermomechanical pulp (TMP); decomposed waste paper pulp (DIP) produced from brown waste paper, kraft envelope waste paper, magazine waste paper, newspaper waste paper, flyer waste paper, office waste paper, cardboard waste paper, white waste paper, Kent waste paper, imitation waste paper, and land certificate waste paper; and chemically or mechanically produced pulps from non-wood fibers such as kenaf, hemp, and reed.
The content of the other pulp may be less than 3 mass%, less than 2 mass%, less than 1 mass%, etc., based on the total mass of the pulp components. One type of other pulp may be used alone, or two or more types may be used in combination.

In the molded paper base material, the freeness of the pulp in the first surface layer is lower than that of the pulp in the middle layer. Also, the freeness of the pulp in the second surface layer is higher than that of the pulp in the middle layer. This allows for the ease of stretch required during mold molding. As a result, excellent processability during mold molding can be achieved.

In one example, it is preferable that the freeness of the pulp in the first surface layer is at least 20 ml lower than the freeness of the pulp in the middle layer, and the freeness of the pulp in the second surface layer is at least 20 ml higher than the freeness of the pulp in the middle layer. By adopting such a configuration, it becomes easier to achieve both processability during molding in a mold and strength of the molded body.

In one example, the freeness of the pulp in the second surface layer of the molded paper substrate is preferably higher than the freeness of the pulp in the first surface layer. By placing the second surface layer on the convex side during mold forming, the ease of stretch required by the convex side can be achieved, further improving processability during mold forming.

When the molded paper substrate has a first underlayer, the freeness of the pulp of the first underlayer is preferably lower than that of the middle layer, and when the molded paper substrate has a second underlayer, the freeness of the pulp of the second underlayer is preferably higher than that of the middle layer.
By adopting such a configuration, it becomes easier to achieve both good processability during molding with a die and good strength of the molded product.

The overall tensile elongation at break of the mold-molding paper substrate is preferably 3.0% or more in the longitudinal direction and 5.0% or more in the transverse direction, and more preferably 3.5% or more in the longitudinal direction and 5.7% or more in the transverse direction.
When the overall tensile elongation at break of the paper substrate for molding in a mold is equal to or greater than the lower limit, it is easy to prevent the substrate from breaking due to insufficient elongation during molding in a mold.

The aspect ratio of the tensile strength of the paper substrate for molding is less than 2. This makes it easier for the press pressure during molding to be evenly distributed lengthwise and widthwise, reducing the occurrence of localized loads. This prevents cracks, tears, and wrinkles due to the misalignment of the surface layer.
The aspect ratio of the tensile strength of the mold forming paper substrate is preferably 1.35 to 1.90, more preferably 1.4 to 1.88, and even more preferably 1.45 to 1.85.
If the aspect ratio of the tensile strength of the paper substrate for molding is less than the lower limit of the above-mentioned range, it is difficult and unrealistic to manufacture the paper.If the aspect ratio of the tensile strength of the paper substrate for molding is more than the upper limit of the above-mentioned range, the pressing pressure during molding becomes uneven, and cracks and wrinkles due to the misalignment of the surface layer are likely to occur.
The aspect ratio of tensile strength is the value of the tensile strength in the machine direction relative to the tensile strength in the cross direction, and is a value determined by the method described in the Examples below.

The aspect ratio of the Taber stiffness of the mold forming paper substrate is preferably 1.2 to 2.4, and more preferably 1.4 to 2.2.
If the aspect ratio of the Taber stiffness of the molded paper substrate is less than the lower limit of the above-mentioned range, it is difficult and unrealistic to manufacture the paper.If the aspect ratio of the Taber stiffness of the molded paper substrate exceeds the upper limit of the above-mentioned range, the press pressure during molding becomes uneven, and cracks and wrinkles due to the displacement of the surface layer are likely to occur.
The aspect ratio of Taber stiffness is the value of the Taber stiffness in the machine direction relative to the Taber stiffness in the cross direction. The aspect ratio of Taber stiffness is a value determined by the method described in the Examples section below.

In the entire paper substrate for molding, the ratio of fibers having a fiber length of more than 2 mm is preferably 5% or less, more preferably 3% or less, and even more preferably 1% or less. When the ratio is equal to or less than the upper limit, the flexibility of the paper substrate for molding is improved, and the processability during molding is further improved.
The abundance ratio of fibers having a fiber length exceeding 2 mm is a value determined by the method described in the Examples section below.

The overall basis weight of the mold forming paper substrate is preferably 270 to 1050 g/ ^m2 , more preferably 400 to 940 g/ ^m2 , and even more preferably 470 to 830 g/ ^m2 .
If the total basis weight of the molded paper substrate is less than the lower limit of the above numerical range, it is likely to break during molding. If the total basis weight of the molded paper substrate exceeds the upper limit of the above numerical range, the paper becomes too strong and difficult to mold. The total basis weight of the molded paper substrate is a value determined by the method described in the examples below.

The overall thickness of the mold molding paper substrate is preferably 300 to 1150 μm, more preferably 400 to 1060 μm, and even more preferably 550 to 990 μm.
If the total thickness of the paper substrate for molding is less than the lower limit of the above numerical range, the paper will be too soft to ensure the strength required for the molded paper product. If the total thickness of the paper substrate for molding exceeds the upper limit of the above numerical range, the paper will be too hard and difficult to mold.
The total thickness of the mold molding paper substrate is a value determined by the method described in the Examples section below.

The overall density of the mold forming paper substrate is preferably 0.7 to 1.0 g/cm ³ , more preferably 0.72 to 0.95 g/cm ³ , and even more preferably 0.75 to 0.9 g/cm ³ .
If the overall density of the molded paper substrate is below the lower limit of the above range, it is difficult and impractical to produce paper, whereas if the overall density of the molded paper substrate is above the upper limit of the above range, the paper is too hard and difficult to process.
The overall density of the mold forming paper substrate is a value determined by the method described in the Examples section below.

The overall interlaminar strength of the mold forming paper substrate is preferably 820 kPa or more, more preferably 1040 kPa or more, and even more preferably 1270 kPa or more.
When the overall interlaminar strength of the paper substrate for molding with a mold is equal to or greater than the lower limit of the aforementioned numerical range, the occurrence of partial swelling of the paper substrate for molding with a mold can be suppressed during molding with a mold.
The overall interlaminar strength of the mold-molded paper substrate is a value determined by the method described in the examples below.

The Cobb sizing degree 2 minutes of the mold forming paper substrate is preferably less than 38 g, more preferably less than 36 g, and even more preferably less than 34 g.
If the Cobb sizing degree of the mold-molded paper base material is within the above-mentioned numerical range, it can have good water resistance as a paper container for liquids. If the Cobb sizing degree of the mold-molded paper base material exceeds the upper limit of the above-mentioned numerical range, the paper substrate for mold-molded paper absorbs a large amount of water from its surface, which is undesirable because it impairs the strength required for a liquid container.
The Cobb sizing degree of 2 minutes of the paper substrate for mold molding is a value determined by the method described in the examples below.

Of the first and second surface layers, the Oken smoothness of the surface to be printed is preferably 10 seconds or more, more preferably 12 seconds or more, and even more preferably 15 seconds or more.
If the Oken smoothness is less than the lower limit of the above range, the printing characteristics are deteriorated.
The Oken smoothness is a value determined by the method described in the Examples section below.

In one example, the molded paper substrate is particularly preferably a multi-layer paper having a six-layer structure with a first surface layer, a first underlayer, a first middle layer, a second middle layer, a second underlayer, and a second surface layer in this order. In this case, the preferred freeness of each layer is as follows:
The freeness of the first surface layer is preferably from 280 to 480 ml, more preferably from 290 to 460 ml, and even more preferably from 300 to 440 ml.
The freeness of the first subsurface layer is preferably from 290 to 500 ml, more preferably from 300 to 480 ml, and even more preferably from 310 to 460 ml.
The freeness of the first intermediate layer is preferably from 300 to 520 ml, more preferably from 310 to 500 ml, and even more preferably from 320 to 480 ml.
The freeness of the second intermediate layer is preferably from 300 to 520 ml, more preferably from 310 to 500 ml, and even more preferably from 320 to 480 ml.
The freeness of the second subsurface layer is preferably from 310 to 540 ml, more preferably from 320 to 520 ml, and even more preferably from 330 to 500 ml.
The freeness of the second surface layer is preferably from 320 to 600 ml, more preferably from 330 to 580 ml, and even more preferably from 340 to 560 ml.
When the freeness of each layer is within the above numerical range, in a six-layer paper having a first surface layer, a first subsurface layer, a first middle layer, a second middle layer, a second subsurface layer, and a second surface layer in that order, it is easy to achieve both processability during molding in a mold and strength of the molded body.

(Mechanism of action)
In the above-described molded paper base material, the freeness of the pulp in the first surface layer is lower than that of the pulp in the middle layer, and the freeness of the pulp in the second surface layer is higher than that of the pulp in the middle layer. Therefore, by placing the second surface layer side, which is more susceptible to plastic deformation, on the convex surface side of the press, moldability is improved.
In addition, the aspect ratio of the tensile strength of the paper substrate for molding is less than 2. This allows the press pressure during molding to be evenly distributed lengthwise and widthwise, reducing the occurrence of localized loads. This in turn reduces the occurrence of cracks, tears, and wrinkles due to the misalignment of the surface layer.
Therefore, the mold-molded paper substrate of the present invention has excellent processability when obtaining a molded paper product having sufficient strength for practical use.

<Method of manufacturing the paper substrate for mold molding>
The method for producing the mold-molding paper substrate will be described below.
The method for producing a paper substrate for mold molding of the present invention, which has at least a first surface layer, a middle layer and a second surface layer in this order, is characterized in that a first papermaking layer and a second papermaking layer are laminated together.

The first papermaking layer has at least a first surface papermaking layer that becomes the first surface layer after drying. The first surface papermaking layer is obtained by making a pulp slurry for the first surface layer, which contains pulp with a lower freeness than the pulp in the middle layer, using a short-net multi-cylinder papermaking machine.

The second papermaking layer has at least a second surface papermaking layer that becomes the second surface layer after drying. The second surface papermaking layer is obtained by making a pulp slurry for the second surface layer, which contains pulp with a higher freeness than the pulp in the middle layer, using a short-net multi-cylinder papermaking machine.

A multi-cylinder short-wire papermaking machine is a type of papermaking machine that typically uses several sets of short wires installed horizontally in the papermaking direction in the wire part of a single papermaking machine to produce wet paper layer by layer, which is then squeezed in a set of press parts to produce a single sheet of paperboard and dried. This is to produce paperboard that is stronger and has less difference in strength in the length and width directions than a cylinder papermaking machine.
The molded paper base material of the present invention is produced using a short wire multi-cylinder paper machine. The molded paper base material produced using a short wire multi-cylinder paper machine has the characteristic that the fiber orientation of the pulp fibers in each layer is smaller than that produced using a cylinder paper machine.

The molded paper base material has a middle layer. Therefore, at least one of the first papermaking layer and the second papermaking layer has a multi-layer structure. When the first papermaking layer and the second papermaking layer are combined, they are combined so that the first surface papermaking layer and the second surface papermaking layer are respectively positioned on the outer surfaces. At this time, the papermaking layer positioned on the inside becomes the middle layer after drying.

The first papermaking layer may further include a first subsurface papermaking layer that becomes the first subsurface layer after drying. The first subsurface papermaking layer is obtained by making a pulp slurry for the first subsurface layer using a short wire multi-cylinder papermaking machine. The pulp slurry for the first subsurface layer preferably contains pulp that has a lower freeness than the pulp of the middle layer.

The first papermaking layer may further include a first intermediate layer papermaking layer that becomes the intermediate layer after drying. The first intermediate layer papermaking layer is obtained by making the first intermediate layer pulp slurry with a short wire multi-cylinder papermaking machine.
When the first papermaking layer has a first subsurface papermaking layer and a first intermediate papermaking layer, the first papermaking layer is obtained by combining the pulp slurries for the respective papermaking layers one by one in order.

The second papermaking layer may further include a second subsurface papermaking layer that becomes the second subsurface layer after drying. The second subsurface papermaking layer is obtained by making the pulp slurry for the second subsurface layer with a short wire multi-cylinder papermaking machine. The pulp slurry for the second subsurface layer preferably contains pulp with a higher freeness than the pulp of the middle layer.

The second papermaking layer may further include a second intermediate layer papermaking layer that becomes the intermediate layer after drying. The second intermediate layer papermaking layer is obtained by making the second intermediate layer pulp slurry with a short wire multi-cylinder papermaking machine.
When the second papermaking layer has a second subsurface papermaking layer and a second middle papermaking layer, the second papermaking layer is obtained by combining the pulp slurries for the respective papermaking layers one by one in order.

The freeness of the pulp slurry for the first surface layer is preferably lower than the freeness of the pulp slurry for the second surface layer. By placing the second surface layer obtained after drying on the convex side during molding in a die, the ease of stretching required by the convex side can be achieved. This further improves the processability during molding in a die.

The layer structure of the paper substrate for molding and the details and preferred aspects of each layer are the same as those explained in the section entitled "Paper substrate for molding".
In one example, a six-layer molded paper substrate can be manufactured having a first surface layer, a first undersurface layer, a first middle layer, a second middle layer, a second undersurface layer, and a second surface layer in this order. When obtaining a molded paper substrate as a multi-layer paper substrate having a six-layer structure, a first paper layer having a first surface layer, a first undersurface layer, and a first middle layer in this order; and a second paper layer having a second middle layer, a second undersurface layer, and a second surface layer in this order; can be combined.
At this time, the first intermediate layer and the second intermediate layer are laminated together to form a two-layer intermediate layer, so that the first surface layer and the second surface layer are disposed on the outer surfaces of the respective layers.

In addition, the freeness of the pulp slurry of each layer preferably satisfies the condition "first surface layer" < "first subsurface layer" < "first middle layer" < "second middle layer" < "second subsurface layer" < "second surface layer" so that dewatering is performed well during the papermaking process.

The pulp slurry of each layer is a papermaking slurry containing at least cellulose pulp and water. The cellulose pulp may be beaten before or after the preparation of the pulp slurry of each layer. The beating conditions are adjusted for each slurry of each layer to achieve a desired freeness value.
The preferred range of the freeness of each layer is the same as that explained in the section <Paper substrate for mold molding>.

The pulp slurry of each layer may further contain various fillers as necessary. There are no particular limitations on the fillers. Examples include inorganic fillers such as clay, calcined kaolin, delaminated kaolin, 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; and organic fillers such as urea-formaldehyde resin, polystyrene resin, phenolic resin, and microhollow particles. One type of filler may be used alone, or two or more types may be used in combination.

The pulp slurry of each layer may further contain various internal additives used in normal papermaking as necessary. The internal additives are not particularly limited. Examples of the internal additives include sizing agents, retention improvers, freeness improvers, paper strength enhancers, wet paper strength enhancers, starch, starches such as cationic starch, bulk enhancers, thickeners, aluminum sulfate, polyvalent metal compounds, silica sol, defoamers, coloring dyes, coloring pigments, fluorescent brighteners, pH adjusters, pitch control agents, and slime control agents. For example, internal sizing agents such as alkyl ketene dimers, styrene acrylic resins, and rosin are used as sizing agents. In addition, polyacrylamide resins, polyamide epichlorohydrin resins, polyethyleneimines and derivatives thereof, polyethylene oxides, polyamines, polyamides, polyamide polyamines and derivatives thereof, cationic and amphoteric starches, oxidized starches, carboxymethylated starches, vegetable gums, and organic compounds such as polyvinyl alcohol can be used in appropriate combination as paper strength enhancers and retention improvers. These additives can also be added by spraying between paper layers during the papermaking process, or by coating the surface of the paper base material molded in a mold during or after papermaking.
The internal additive assistants may be used alone or in combination of two or more kinds.

When a molded paper base material is used as a food packaging container, some foods are highly acidic. Highly acidic foods may cause the paper to deteriorate. In this case, it may be beneficial to use an alkyl ketene dimer as a sizing agent.

(Mechanism of action)
In the manufacturing method of the molded paper base material described above, the freeness of the pulp in the first surface layer is lower than that of the pulp in the middle layer, and the freeness of the pulp in the second surface layer is higher than that of the pulp in the middle layer, which improves the dewatering property of the papermaking process and improves operability.

Conventionally, paper base materials have been made using a cylinder-type papermaking machine. When a cylinder-type papermaking machine is used, the flexibility in the longitudinal direction is insufficient. In contrast, by making paper using a short-wire papermaking machine as in the present invention, the aspect ratio of the tensile strength is smaller than that when a cylinder-type papermaking machine is used, being less than 2. Therefore, the press pressure during molding is evenly distributed lengthwise and widthwise, thereby reducing the occurrence of localized loads. Therefore, the occurrence of cracks, breaks, and wrinkles due to the shifting of the surface layer can be suppressed.
In addition, the manufacturing method of the present invention adopts a multi-layer papermaking method instead of a single-layer papermaking method. Therefore, the misalignment between layers is mitigated against the shear stress applied in the thickness direction of the paper during deep drawing. Therefore, the paper base material is easily conformed to the mold used for deep drawing during molding.
Therefore, according to the manufacturing method of the present invention, a mold-molded paper base material can be obtained that has excellent processability when producing a molded paper product that has sufficient strength for practical use.

Because the paper base material for mold forming is obtained by combining materials, its thickness can be freely adjusted. Therefore, when multiple sheets of paper base material for mold forming are glued together, it is easy to reduce the number of sheets to be glued together. As a result, it is expected that the amount of glue used and drying time will be reduced.

<Applications of paper substrate for mold molding>
The molded paper substrate can be suitably used in mold forming to obtain various molded paper products. In mold forming, it is preferable to arrange the second surface layer on the convex surface side during mold forming. This is because the second surface layer, which contains pulp with a relatively high freeness, is more easily plastically deformed and has excellent flexibility than the first surface layer.

There is no particular use for the molded paper base material. Suitable examples include deep-drawn paper plates, paper cutlery, and paper hangers. Other examples include relatively deep paper trays and paper cups.

Although several embodiments have been described above, the present invention is not limited to the embodiments disclosed in this specification, and can be modified as appropriate without departing from the spirit of the invention. The embodiments disclosed in this specification can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention.

The present invention will be specifically described below using examples, but the present invention is not limited to the following description.

Example 1
(Preparation of pulp slurry for first surface layer)
A pulp slurry containing 100% LBKP beaten to a freeness of 325 ml was obtained as a pulp component for the first surface layer. 0.7% by mass of a paper strength agent (PAM-based paper strength agent), 0.1% by mass of cationic starch, 0.1% by mass of a rosin-based internal sizing agent, and 1.0% by mass of aluminum sulfate as a fixing agent were added to the obtained pulp slurry, each of which was based on 100% by mass of the dry mass of the pulp, to prepare a pulp slurry for the first surface layer.

(Preparation of pulp slurry for second surface layer)
A pulp slurry containing 100% LBKP beaten to a freeness of 456 ml was obtained as a pulp component for the second surface layer. 0.7% by mass of a paper strength agent (PAM-based paper strength agent), 0.1% by mass of cationic starch, 0.3% by mass of a rosin-based internal sizing agent, and 1.3% by mass of aluminum sulfate as a fixing agent were added to the obtained pulp slurry, each of which was based on 100% by mass of the dry mass of the pulp, to prepare a pulp slurry for the second surface layer.

(Preparation of pulp slurry for first subsurface papermaking layer)
A pulp slurry of 100% LBKP beaten to a freeness of 366 ml was obtained as a pulp component for the first undersurface papermaking layer. 0.4% by mass of a paper strength agent (PAM-based paper strength agent), 0.1% by mass of cationic starch, 0.4% by mass of a rosin-based internal sizing agent, and 1.0% by mass of aluminum sulfate as a fixing agent were added to the obtained pulp slurry, each based on 100% by mass of the dry mass of the pulp, to prepare a pulp slurry for the first undersurface papermaking layer.

(Preparation of pulp slurry for second subsurface papermaking layer)
A pulp slurry of 100% LBKP beaten to a freeness of 430 ml was obtained as a pulp component for the second undersurface papermaking layer. 0.5% by mass of a paper strength agent (PAM-based paper strength agent), 0.1% by mass of cationic starch, 0.4% by mass of a rosin-based internal sizing agent, and 1.2% by mass of aluminum sulfate as a fixing agent were added to the obtained pulp slurry, each based on 100% by mass of the dry mass of the pulp, to prepare a pulp slurry for the second undersurface papermaking layer.

(Preparation of pulp slurry for first intermediate layer)
A pulp slurry containing 100% LBKP beaten to a freeness of 385 ml was obtained as a pulp component for the first intermediate layer. 0.4% by mass of a paper strength agent (PAM-based paper strength agent), 0.4% by mass of a rosin-based internal sizing agent, and 1.1% by mass of aluminum sulfate as a fixing agent were added to the obtained pulp slurry, each of which was based on 100% by mass of the dry mass of the pulp, to prepare a pulp slurry for the first intermediate layer.

(Preparation of pulp slurry for second intermediate layer)
A pulp slurry containing 100% LBKP beaten to a freeness of 393 ml was obtained as a pulp component for the second intermediate layer. 0.4% by mass of a paper strength agent (PAM-based paper strength agent), 0.4% by mass of a rosin-based internal sizing agent, and 1.1% by mass of aluminum sulfate as a fixing agent were added to the obtained pulp slurry, each of which was based on 100% by mass of the dry mass of the pulp, to prepare a pulp slurry for the second intermediate layer.

(Papermaking)
The prepared pulp slurries were each made into paper using a short wire multi-cylinder paper machine, and each paper layer was dried to obtain a 6-layered molded paper base material having a basis weight of 751 g/m ² and a paper thickness of 913 μm.

Example 2
A mold-molding paper substrate having a basis weight of 708 g/m ² and a thickness of 952 μm was obtained by the same operation and method as in Example 1, except that the aspect ratio of the tensile strength in Example 1 was changed to 1.42.

Example 3
The same operations and methods as in Example 1 were carried out, and further a smoothing treatment was carried out by a machine calender to obtain a mold-molding paper substrate having a basis weight of 635 g/m ² and a paper thickness of 756 μm.

<Comparative Example 1>
A mold-molded paper substrate was obtained in which the second surface layer of the mold-molded paper substrate A of Example 1 was placed on the convex side during mold molding.

<Comparative Example 2>
In Example 3, the pulp used for the first and second surface layers was beaten to a freeness of 450 ml to obtain a pulp slurry containing 39% NBKP and 61% LBKP. 0.9% by mass of a paper strength agent (PAM-based paper strength agent), 0.6% by mass of a rosin-based internal sizing agent, and 1.9% by mass of aluminum sulfate as a fixing agent were added to the obtained pulp slurry, each relative to 100% by mass of the dry mass of the pulp, to prepare the pulp slurries for the first and second surface layers.

The pulp components used for the first subsurface paper layer, the first middle layer paper layer, the second middle layer paper layer, and the second subsurface paper layer were beaten to a freeness of 420 ml to obtain a pulp slurry with a pulp blend of 30% NBKP and 70% LBKP. 0.5% by mass of a paper strength enhancer (PAM-based paper strength enhancer), 0.5% by mass of a rosin-based internal sizing agent, and 1.5% by mass of aluminum sulfate as a fixing agent were added to the obtained pulp slurry, each relative to 100% by mass of the dry mass of the pulp, to prepare the pulp slurries for the first subsurface paper layer, the first middle layer paper layer, the second middle layer paper layer, and the second subsurface paper layer.

Except for the preparation of the pulp slurry for each layer, the same operations and methods as in Example 3 were used to obtain a mold-molded paper base material having a basis weight of 412 g/m ² and a paper thickness of 470 μm.

<Comparative Example 3>
In Example 3, the short wire multi-cylinder papermaking machine was changed to a round wire multi-cylinder papermaking machine, a five-layer structure was formed having a first surface papermaking layer, a first subsurface papermaking layer, a middle layer papermaking layer, a second subsurface papermaking layer, and a second surface papermaking layer in that order, and the basis weight, paper thickness, and freeness of the pulp slurry of each layer were changed according to Table 1. The same procedure was followed as in Example 3, and a mold-molded paper base material with a basis weight of 416 g/ ^m2 and a paper thickness of 426 μm was obtained.

<Comparative Example 4>
Using a cylinder-net multi-cylinder papermaking machine, a six-layer structure was made having a first surface paper layer, a first subsurface paper layer, a first middle layer paper layer, a second middle layer paper layer, a second subsurface paper layer, and a second surface paper layer in that order.

The pulp used for the first and second surface layers was beaten to a freeness of 385 ml to obtain a pulp slurry with a pulp blend of 13% NBKP and 87% LBKP. 3.0% by mass of a paper strength enhancer (PAM-based paper strength enhancer) was added to the obtained pulp slurry per 100% by mass of the dry mass of the pulp to prepare the pulp slurries for the first and second surface layers.

As the pulp components for the first subsurface papermaking layer, the first middle layer papermaking layer, the second middle layer papermaking layer, and the second subsurface papermaking layer, a pulp slurry with a pulp blend of 37% NBKP and 63% LBKP was obtained by beating to a freeness of 380 ml. 3.0% by mass of a paper strength enhancer (PAM-based paper strength enhancer) and 1.5% by mass of an alkyl ketene dimer-based internal sizing agent were added to the obtained pulp slurry, respectively, relative to 100% by mass of the dry mass of the pulp, to prepare pulp slurries for the first subsurface papermaking layer, the first middle layer papermaking layer, the second middle layer papermaking layer, and the second subsurface papermaking layer.

(Papermaking)
The prepared pulp slurries were each made into paper using a cylinder-net multi-cylinder paper machine, and each paper layer was dried to obtain a 6-layered molded paper base material having a basis weight of 416 g/m ² and a paper thickness of 480 μm.

<Measurement method>
For each example of the mold-molding paper substrate, various physical properties were measured as described below.

(Freeness of each layer)
The freeness of each layer of the base paper was measured using a Canadian Standard Freeness Tester in accordance with JIS P 8121-2:2012 after disintegrating the sample using a standard disintegrator in accordance with JIS P 8220-2:2012.

(Total basis weight of mold forming paper substrate)
The basis weight of the base paper was measured in accordance with JIS P 8124:2011.

(Total thickness of molded paper substrate)
The thickness of the base paper was measured in accordance with JIS P 8118:2014.

(Overall density of molded paper substrate)
The density of the base paper was calculated from the above basis weight and paper thickness.

(Aspect ratio of tensile strength)
The tensile strength in the machine direction and the tensile strength in the cross direction of the base paper were measured in accordance with JIS P 8113:2006, and the ratio of the tensile strength in the machine direction to the tensile strength in the cross direction was calculated from the measured values.

(Tensile elongation at break)
The tensile elongation at break of the base paper was measured in accordance with JIS P 8113:2006.

(Taber stiffness)
The Taber stiffness of the base paper was measured in accordance with JIS P 8125:2000, and the value in the machine direction relative to the value in the cross direction was calculated.

(Overall interlaminar strength of molded paper substrate)
The interlaminar strength of the base paper was measured in accordance with JIS P 8131:2009.

(Oken type smoothness)
The Oken smoothness of the base paper was measured in accordance with JIS P 8155:2010.

(Hump size of molded paper substrate)
The cob size of the base paper was measured in accordance with JIS P 8140:1998.

<Evaluation method>
The molded paper substrate of each example was conditioned for 72 hours in an environment of 30°C and 90% RH, then cut into B5 size pieces and cut into arcs with a radius of 5 cm at the four corners to obtain blanks. A lunchbox-shaped mold was used, and the second surface layer side of the blank was placed on the female mold side and squeezed to obtain a paper molded body with a depth of 24 mm, long sides of 264 mm, and short sides of 113 mm.

The molded paper products obtained were evaluated for their suitability for drawing as described below. Evaluation results of ◯ and Δ were judged to be acceptable. The evaluation results are shown in Table 1.
(Processability during drawing: length of crack at corner)
The length of the crack in the rounded corner of the molded paper product was evaluated according to the following criteria.
A: No cracks are observed in the rounded corners.
Δ: The length of the crack at the R portion of the corner is 10 mm or less.
×: The length of the crack at the R portion of the corner exceeds 10 mm.

(Processability during drawing: width of cracks at corners)
A: No cracks are observed in the rounded corners.
Δ: Cracks are observed at the R portion of the corner, but they are not linear tears.
×: The R part of the corner is torn or appears to be torn.

In Examples 1 to 3, molded paper substrates were obtained that had excellent workability when obtaining molded paper bodies with sufficient strength for practical use. In contrast, in Comparative Examples 1 to 4, it was not possible to achieve both sufficient strength for practical use and workability during drawing.

The present invention provides a molded paper substrate that is easy to process and has sufficient strength for practical use, and a method for producing the same.

Claims (4)

A mold forming paper substrate having at least a first surface layer, a middle layer, and a second surface layer in this order,
The freeness of the pulp of the first surface layer is lower than the freeness of the pulp of the middle layer;
The freeness of the pulp of the second surface layer is higher than the freeness of the pulp of the middle layer,
A molded paper substrate having a tensile strength aspect ratio of less than 2. The molded paper substrate according to claim 1, wherein the freeness of the pulp in the first surface layer is lower than the freeness of the pulp in the second surface layer. A method for producing a mold-molded paper substrate having at least a first surface layer, a middle layer, and a second surface layer in this order, comprising:
A first papermaking layer having at least a first surface papermaking layer obtained by making a pulp slurry for a first surface layer containing a pulp having a lower freeness than the pulp of the middle layer using a short wire multi-cylinder papermaking machine;
A second papermaking layer having at least a second surface papermaking layer obtained by making a pulp slurry for a second surface layer containing pulp having a higher freeness than the pulp of the middle layer using a short wire multi-cylinder papermaking machine;
A manufacturing method comprising: The manufacturing method according to claim 3, wherein the freeness of the pulp slurry for the first surface layer is lower than the freeness of the pulp slurry for the second surface layer.