Mold-molded paper substrate, method for manufacturing mold-molded paper substrate

Abstract

To provide a die molding paper material that is superior in processability when a paper molded body having practically sufficient strength is obtained, and a manufacturing method thereof.SOLUTION: A die molding paper material includes at least a first surface layer, a middle layer and a second surface layer in this order. Freeness of pulp of the first surface layer is lower than freeness of pulp of the middle layer. Freeness of pulp of the second surface layer is higher than the freeness of the pulp of the middle layer. The die molding paper material has an aspect ratio of tensile strength of less than 2.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to a mold-molded paper substrate and a method for manufacturing a mold-molded paper substrate. [Background technology]

[0002] Traditionally, plastic containers have been used for food packaging such as lunch boxes and food trays, as well as for packaging industrial products such as CD cases and electrical component cases. In recent years, as a measure against environmental problems, it has been proposed to replace these plastic containers with paper molded products made from natural pulp (for example, Patent Document 1).

[0003] Paper molded products can take on a variety of shapes, from shallow plates and cups with almost no drawing depth to molded products with considerable drawing depth. Therefore, there is a need to manufacture paper molded products of various shapes from a paper substrate. Generally, when forming a molded body with a deep draw depth, the paper substrate is stretched more than when the draw depth is shallow. Also, the outer paper layer of the molded body is stretched more than the inner paper layer during the draw molding process. Therefore, the outer paper layer of a molded body with a deep draw depth is more prone to tearing during molding.

[0004] Patent Document 1 proposes that the elongation at break of the outer paper layer of the molded body be set to 5% or more in order to prevent the paper from tearing. Furthermore, Patent Document 1 suggests that in order to obtain a paper molded body with high rigidity during deep drawing, the overall density of the paper substrate should be 0.4 to 0.7 g / cm³. 3 It has been proposed that this be done. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2002-200726 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] However, according to the inventors' research, in the case of the paper substrate proposed in Patent Document 1, when a molded body with a deep draw depth is formed in a mold, cracks, tears, and wrinkles due to displacement of the surface layer occur, indicating room for improvement in processability. Furthermore, it is difficult to achieve both processability during deep drawing and strength of the molded body. Keeping the overall density of the paper substrate within the aforementioned numerical range is not practical due to the high difficulty of operation.

[0007] The present invention provides a mold-molded paper substrate and a method for manufacturing the same, which offer excellent processability when obtaining a paper molded article with sufficient strength for practical use. [Means for solving the problem]

[0008] The inventors have found that the above problem can be solved by using a short-wire multi-cylinder papermaking machine and optimizing the freeness of the pulp in each layer of multi-layer papermaking for mold molding.

[0009] The present invention has the following aspects. [1] A mold-molded paper substrate having at least a first surface layer, a middle layer and a second surface layer in this order; the pulp freeness of the first surface layer is lower than that of the middle layer; the pulp freeness of the second surface layer is higher than that of the middle layer; and the aspect ratio of the tensile strength is less than 2. [2] The mold-molded paper substrate according to [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. [3] A method for manufacturing a mold-formed paper substrate having at least a first surface layer, a middle layer and a second surface layer in this order; characterized by combining a first paper layer having at least a first surface paper layer obtained by papermaking a pulp slurry for the first surface layer containing pulp with lower freeness than the pulp of the middle layer in a short-wire multi-cylinder paper machine; and a second paper layer having at least a second surface paper layer obtained by papermaking a pulp slurry for the second surface layer containing pulp with higher freeness than the pulp of the middle layer in a short-wire multi-cylinder paper machine. [4] The manufacturing method according to [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. [Effects of the Invention]

[0010] According to the present invention, a mold-molded paper substrate with excellent processability for obtaining a paper molded article having sufficient strength for practical use is provided, as well as a method for manufacturing the same. [Modes for carrying out the invention]

[0011] "Freeness" refers to disintegration freeness (CSF). Disintegration freeness refers to the Canadian standard filtration rate measured using a pulp slurry obtained by disintegrating a paper substrate. In other words, freeness is the filtration rate measured using a Canadian standard filtration rate tester in accordance with JIS-P8121-2:2012, after the sample has been disintegrated using a standard disintegrator in accordance with JIS-P8220-2:2012. "Transverse direction" is defined as the direction perpendicular to the direction of travel of the paper machine on the surface of the paper substrate. The direction of travel of the paper machine can be determined from the orientation direction of the fibers in the paper substrate. "Longitudinal direction" is the direction perpendicular to the transverse direction on the surface of the paper substrate. The "~" symbol, which indicates a numerical range, means that the numbers before and after it are included as the lower and upper limits, respectively. The lower and upper limits of the numerical ranges disclosed herein can be combined in any way to create new numerical ranges.

[0012] Several embodiments are described in detail below. The following disclosures relate to the description of representative embodiments, and the present invention is not limited to these disclosures.

[0013] <Mold-molded paper base material> The mold-formed paper substrate of the present invention is a multilayer-formed paper having at least a first surface layer, a middle layer, and a second surface layer in that order. The mold-formed paper substrate of the present invention may be a three-layer multilayer-formed paper, or it may be a multilayer-formed paper with about four to nine layers.

[0014] The first surface layer is the layer arranged on the outermost side in the multi-layered mold-formed paper base material. The second surface layer is the layer arranged on the innermost side (opposite side to the first surface layer) in the multi-layered mold-formed paper base material. That is, the first surface layer and the second surface layer are the outermost layers of the mold-formed paper base material. Therefore, during mold forming, the first surface layer and the second surface layer directly contact the mold. The raw material compositions of each of the first surface layer and the second surface layer may be the same or different from each other. Also, the raw material compositions of each of the first surface layer and the second surface layer may be the same as or different from those of the middle layer.

[0015] The middle layer is the layer arranged 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 compositions of each layer of the middle layer may be the same or different.

[0016] The mold-formed paper base material may or may not have a first sub-surface layer that contacts the first surface layer between the first surface layer and the middle layer. When the middle layer has a multi-layer structure of three or more layers, the layer immediately below the first surface layer is taken as the first sub-surface layer. It can also be said that the first sub-surface layer is the layer that contacts the first surface layer inside the mold-formed paper base material.

[0017] The mold-formed paper base material may or may not have a second sub-surface layer that contacts the second surface layer between the second surface layer and the middle layer. When the middle layer has a multi-layer structure of three or more layers, the layer immediately below the second surface layer is taken as the second sub-surface layer. It can also be said that the second sub-surface layer is the layer that contacts the second surface layer inside the mold-formed paper base material.

[0018] Each layer of the mold-formed paper base material has cellulose pulp as the main component. The main component of each layer refers to the component that occupies 50% by mass or more among 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 portion of the cellulose pulp in each layer may be of the same type, or they may be different from each other. Since improved flexibility further enhances processability during mold molding, it is particularly preferable that all layers of the mold-molded paper substrate contain LKP as cellulose pulp.

[0019] Cellulose pulp may be bleached or unbleached. Unless otherwise specified, LKP and NKP are terms that include bleached pulp and unbleached pulp, respectively. Bleached hardwood kraft pulp is sometimes written as "LBKP". Bleached softwood kraft pulp is sometimes written as "NBKP". Examples of LKP wood include acacia and eucalyptus. Examples of NKP wood include radiata pine. Cellulose pulp may be used alone or in combination of two or more types.

[0020] Each layer of the molded paper substrate may contain pulp other than NKP and LKP. Examples of other pulp include mechanical pulp such as stone ground pulp (SGP), pressure stone ground pulp (PGW), refiner ground pulp (RGP), thermo ground pulp (TGP), chemiground pulp (CGP), wood pulp (GP), and thermomechanical pulp (TMP); disintegrated recycled paper pulp (DIP) produced from recycled brown paper, recycled kraft envelopes, recycled magazines, recycled newspapers, recycled flyers, recycled office paper, recycled corrugated cardboard, recycled white paper, recycled Kent paper, recycled imitation paper, recycled land certificate paper, etc.; and pulp produced chemically or mechanically from non-wood fibers such as kenaf, hemp, and reed. The content of other pulps may be less than 3% by mass, less than 2% by mass, or less than 1% by mass, etc., relative to 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.

[0021] In die-molded paper substrates, the freeness of the pulp in the first surface layer is lower than that of the pulp in the middle layer. Furthermore, the freeness of the pulp in the second surface layer is higher than that of the pulp in the middle layer. Therefore, the elongation required during die molding can be achieved. As a result, excellent processability during die molding can be realized.

[0022] In one example, it is preferable that the freeness of the pulp in the first surface layer is 20 ml or more lower than the freeness of the pulp in the middle layer, and that the freeness of the pulp in the second surface layer is 20 ml or more higher than the freeness of the pulp in the middle layer. By adopting such a configuration, it becomes easier to achieve both processability during mold molding and strength of the molded article.

[0023] In one example, it is preferable that the freeness of the pulp in the second surface layer of the mold-molded paper substrate is higher than that of the pulp in the first surface layer. By positioning the second surface layer on the convex side during mold molding, the elongation required for the convex side can be achieved, thus further improving the processability during mold molding.

[0024] When a mold-formed paper substrate has a first subsurface layer, it is preferable that the freeness of the pulp in the first subsurface layer is lower than the freeness of the pulp in the middle layer. Furthermore, when a mold-formed paper substrate has a second subsurface layer, it is preferable that the freeness of the pulp in the second subsurface layer is higher than the freeness of the pulp in the middle layer. By adopting this configuration, it becomes easier to achieve both processability during mold forming and strength of the molded product.

[0025] The overall tensile elongation at break of the mold-molded 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. If the overall tensile elongation at break of the mold-molded paper substrate is above the aforementioned lower limit, it is easier to prevent it from tearing due to insufficient elongation during mold molding.

[0026] The aspect ratio of the tensile strength of the mold-molded paper substrate is less than 2. Therefore, the press pressure during mold molding is more easily distributed evenly in both directions, reducing the occurrence of localized stress. This prevents cracking, tearing, and wrinkles caused by surface layer displacement. The aspect ratio of the tensile strength of the mold-molded 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 a mold-formed paper substrate is below the lower limit of the aforementioned numerical range, it is difficult and impractical to manufacture paper. If the aspect ratio of the tensile strength of a mold-formed paper substrate exceeds the upper limit of the aforementioned numerical range, the press pressure during mold forming becomes uneven, making it prone to cracking and wrinkles due to displacement of the surface layer. The aspect ratio of tensile strength is the ratio of tensile strength in the longitudinal direction to tensile strength in the transverse direction. The aspect ratio of tensile strength is determined by the method described in the examples below.

[0027] The aspect ratio of the tapered stiffness of the mold-molded 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 mold-formed paper substrate is below the lower limit of the aforementioned numerical range, it is difficult and impractical to manufacture paper. If the aspect ratio of the Taber stiffness of the mold-formed paper substrate exceeds the upper limit of the aforementioned numerical range, the press pressure during mold forming becomes uneven, making it easier for cracks and wrinkles due to surface layer displacement to occur. The aspect ratio of the Taber stiffness is the ratio of the vertical Taber stiffness to the horizontal Taber stiffness. The aspect ratio of the Taber stiffness is determined by the method described in the examples below.

[0028] In the entire mold-molded paper substrate, the proportion of fibers with a fiber length exceeding 2 mm is preferably 5% or less, more preferably 3% or less, and even more preferably 1% or less. When this proportion is below the above upper limit, the flexibility of the mold-molded paper substrate is improved, resulting in improved processability during mold molding. The proportion of fibers with a fiber length exceeding 2 mm is determined by the method described in the examples below.

[0029] The overall basis weight of the mold-molded paper substrate is 270-1050 g / m². 2 Preferably, 400-940 g / m² 2 More preferably, 470-830 g / m 2 That is even more preferable. If the total basis weight of the mold-molded paper substrate is below the lower limit of the aforementioned numerical range, tearing is likely to occur during mold molding. If the total basis weight of the mold-molded paper substrate exceeds the upper limit of the aforementioned numerical range, the paper becomes too rigid, making molding difficult. The total basis weight of the mold-molded paper substrate is a value obtained by the method described in the examples below.

[0030] The overall thickness of the mold-molded paper substrate is preferably 300 to 1150 μm, more preferably 400 to 1060 μm, and even more preferably 550 to 990 μm. If the overall thickness of the molded paper substrate is below the lower limit of the aforementioned numerical range, the paper is too soft and cannot secure the strength required for the molded paper body. Paper with an overall thickness exceeding the upper limit of the aforementioned numerical range is too hard and difficult to mold. The overall thickness of the mold-molded paper substrate is determined by the method described in the examples below.

[0031] The overall density of the mold-molded paper substrate is 0.7-1.0 g / cm³. 3 Preferably, 0.72-0.95 g / cm³ 3 More preferably, 0.75-0.9 g / cm³ 3 That is even more preferable. If the overall density of the mold-molded paper substrate is below the lower limit of the aforementioned numerical range, it is difficult and impractical to manufacture paper. Paper with an overall density exceeding the upper limit of the aforementioned numerical range becomes too hard, making molding difficult. The overall density of the mold-molded paper substrate is the value obtained by the method described in the examples below.

[0032] The overall interlayer strength of the mold-molded paper substrate is preferably 820 kPa or higher, more preferably 1040 kPa or higher, and even more preferably 1270 kPa or higher. If the overall interlayer strength of the mold-molded paper substrate is greater than or equal to the lower limit of the aforementioned numerical range, the occurrence of partial blistering of the mold-molded paper substrate during mold molding can be suppressed. The overall interlayer strength of the mold-molded paper substrate is determined by the method described in the examples below.

[0033] The bump size of the mold-formed paper substrate is preferably less than 38g, more preferably less than 36g, and even more preferably less than 34g. If the degree of bump size of the mold-molded paper substrate is within the aforementioned numerical range, good water resistance can be obtained for use as a liquid paper container. If the degree of bump size of the mold-molded paper substrate exceeds the upper limit of the aforementioned numerical range, the absorption of moisture from the surface of the mold-molded paper substrate will be large, impairing the strength required for a liquid container, which is undesirable. The bump size degree of 2 min of the mold-molded paper substrate is a value obtained by the method described in the examples below.

[0034] Of the first and second surface layers, the Wangyan 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 Wang Ken smoothness value falls below the lower limit of the aforementioned numerical range, the printing characteristics will deteriorate. The Wang-Ran smoothness value is a value obtained by the method described in the examples below.

[0035] In one example, the mold-formed paper substrate is preferably a multilayer paper having a six-layer structure in which the first surface layer, the first subsurface layer, the first middle layer, the second middle layer, the second subsurface layer, and the second surface layer are in this order. In this case, preferred embodiments of the freeness of each layer are as follows. The freeness of the first surface layer is preferably 280-480 ml, more preferably 290-460 ml, and even more preferably 300-440 ml. The freeness of the first subsurface layer is preferably 290-500 ml, more preferably 300-480 ml, and even more preferably 310-460 ml. The freeness of the first middle layer is preferably 300-520 ml, more preferably 310-500 ml, and even more preferably 320-480 ml. The freeness of the second middle layer is preferably 300-520 ml, more preferably 310-500 ml, and even more preferably 320-480 ml. The freeness of the second surface layer is preferably 310 to 540 ml, more preferably 320 to 520 ml, and even more preferably 330 to 500 ml. The freeness of the second surface layer is preferably 320-600 ml, more preferably 330-580 ml, and even more preferably 340-560 ml. If the freeness of each layer is within the aforementioned numerical range, a multilayer papermaking structure with a six-layer structure 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 makes it easier to achieve both processability during die molding and strength of the molded product.

[0036] (Mechanism of action) In the mold-formed paper substrate described above, the pulp freeness of the first surface layer is lower than that of the middle layer, while the pulp freeness of the second surface layer is higher. Therefore, by positioning the second surface layer, which is more easily plastically deformed, on the convex side of the press, the moldability can be improved. In addition, in die-formed paper substrates, the aspect ratio of tensile strength is less than 2. Therefore, the press pressure during die forming is evenly distributed in both the vertical and horizontal directions, which reduces the occurrence of localized loads. This suppresses the occurrence of cracks, tears, and wrinkles caused by surface layer displacement. Therefore, the mold-molded paper substrate of the present invention exhibits excellent processability when obtaining a paper molded body with sufficient strength for practical use.

[0037] <Method for manufacturing molded paper substrate> The following describes the manufacturing method for mold-molded paper substrates. The present invention relates to a method for manufacturing a mold-formed paper substrate, which has at least a first surface layer, a middle layer, and a second surface layer in that order, characterized in that it involves joining a first papermaking layer and a second papermaking layer.

[0038] 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 papermaking a pulp slurry for the first surface layer containing pulp with lower freeness than the pulp of the middle layer using a short-wire multi-cylinder papermaking machine.

[0039] 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 papermaking a pulp slurry for the second surface layer containing pulp with higher freeness than the pulp of the middle layer using a short-wire multi-cylinder papermaking machine.

[0040] A short-wire multi-cylinder paper machine is typically a paper machine in which wet paper is produced one layer at a time using multiple sets of short wires installed horizontally to the papermaking direction in the wire section of a single paper machine, and then pressed into a single sheet of cardboard using a set of presses and dried. This method produces cardboard that is stronger and has less difference in strength between the warp and weft directions than a cylinder wire paper machine. The mold-formed paper substrate of the present invention is obtained by manufacturing it using a short-wire multi-cylinder paper machine. The mold-formed paper substrate obtained by papermaking with a short-wire multi-cylinder paper machine is characterized by having a smaller fiber orientation of pulp fibers in each layer compared to a cylinder-wire paper machine.

[0041] The mold-formed paper substrate has an intermediate layer. Therefore, at least one of the first papermaking layer and the second papermaking layer has a multilayer structure. When the first papermaking layer and the second papermaking layer are assembled, they are assembled so that the first surface papermaking layer and the second surface papermaking layer are positioned on the outer surface, respectively. In this case, the papermaking layer positioned on the inside becomes the intermediate layer after drying.

[0042] The first papermaking layer may further have a first subsurface papermaking layer that becomes the first subsurface layer after drying. The first subsurface papermaking layer is obtained by papermaking the 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 with lower freeness than the pulp of the middle layer.

[0043] The first papermaking layer may further have a first intermediate papermaking layer that becomes the intermediate layer after drying. The first intermediate papermaking layer is obtained by papermaking the first intermediate layer pulp slurry in a short-wire multi-cylinder papermaking machine. If the first papermaking layer has a first subsurface papermaking layer and a first middle papermaking layer, the first papermaking layer is obtained by sequentially forming one layer of pulp slurry for each papermaking layer.

[0044] The second papermaking layer may further have a sub-surface papermaking layer that becomes the sub-surface layer after drying. The sub-surface papermaking layer is obtained by papermaking the pulp slurry for the sub-surface layer using a short-wire multi-cylinder papermaking machine. The pulp slurry for the sub-surface layer preferably contains pulp with higher freeness than the pulp in the middle layer.

[0045] The second papermaking layer may further have a second intermediate papermaking layer that becomes the middle layer after drying. The second intermediate papermaking layer is obtained by papermaking the pulp slurry for the second intermediate layer using a short-wire multi-cylinder papermaking machine. If the second papermaking layer has a second subsurface papermaking layer and a second middle papermaking layer, the second papermaking layer is obtained by sequentially forming one layer of pulp slurry for each papermaking layer.

[0046] The freeness of the pulp slurry for the first surface layer is preferably lower than that of the pulp slurry for the second surface layer. By positioning the second surface layer obtained after drying on the convex side during mold molding, the elongation required for the convex side can be achieved. As a result, the processability during mold molding is further improved.

[0047] The layer structure of the mold-molded paper substrate, the details of each layer, and preferred embodiments are the same as those described in the section on <Mold-molded paper substrate>. In one example, a mold-formed paper substrate with a six-layer structure can be manufactured, 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. When obtaining a mold-formed paper substrate as a multilayer paper with a six-layer structure in this way, a first paper-making layer having a first surface paper-making layer, a first subsurface paper-making layer, and a first middle paper-making layer in that order; and a second paper-making layer having a second middle paper-making layer, a second subsurface paper-making layer, and a second surface paper-making layer in that order; can be combined. At this time, the first intermediate papermaking layer and the second intermediate papermaking layer are assembled to form a two-layer intermediate structure, thereby positioning the first surface papermaking layer and the second surface papermaking layer on their respective outer surfaces.

[0048] Furthermore, the freeness of the pulp slurry in each layer preferably satisfies the following conditions: "first surface layer" < "first subsurface layer" < "first middle layer" < "second middle layer" < "second subsurface layer" < "second surface layer" in order to ensure good dewatering during the papermaking process.

[0049] Each layer of pulp slurry is a papermaking slurry containing at least cellulose pulp and water. The cellulose pulp may be beaten before or after the preparation of each layer of pulp slurry. By adjusting the beating conditions for each layer of slurry, a desired freeness value can be achieved. The preferred numerical range for the freeness of each layer is the same as described in the section on <Mold-formed paper substrate>.

[0050] Each layer of pulp slurry may further contain various fillers as needed. The fillers are not particularly limited. Examples include inorganic fillers such as clay, calcined kaolin, delaminated kaolin, heavy calcium carbonate, light calcium carbonate, light calcium carbonate-silica composites, magnesium carbonate, barium carbonate, titanium dioxide, zinc oxide, silicon dioxide, amorphous silica, aluminum hydroxide, calcium hydroxide, magnesium hydroxide, and zinc hydroxide; and organic fillers such as urea-formaldehyde resin, polystyrene resin, phenolic resin, and fine hollow particles. One type of filler may be used alone, or two or more types may be used in combination.

[0051] Each layer of pulp slurry may further contain various internal additives used in conventional papermaking, as needed. These internal additives are not particularly limited. Examples include sizing agents, yield enhancers, filtration efficiency enhancers, paper strength enhancers, wet paper strength enhancers, starches such as starch and cationized starch, bulk enhancers, thickeners, aluminum sulfate, polyvalent metal compounds, silica sols, defoamers, coloring dyes, coloring pigments, fluorescent whitening agents, pH adjusters, pitch control agents, and slime control agents. For example, alkyl ketene dimers, styrene acrylic resins, and rosin are used as internal sizing agents. Furthermore, polyacrylamide resins, polyamide epichlorohydrin resins, polyethyleneimines and their derivatives, polyethylene oxide, polyamines, polyamides, polyamide polyamines and their derivatives, cationic and amphoteric starches, oxidized starch, carboxymethylated starch, plant gums, polyvinyl alcohol, and other organic compounds can be used in appropriate combinations as paper strength enhancers and yield enhancers. Furthermore, these can also be added by spraying them between paper layers during the papermaking process, or by coating them onto the surface of the die-formed paper substrate during or after papermaking. Internal medications may be used individually or in combination of two or more.

[0052] When paper molded products made from molded paper substrates are used as food packaging containers, some foods contain high acidity. High acidity can degrade the paper. In such cases, the use of alkyl ketene dimers as sizing agents may be beneficial.

[0053] (Mechanism of action) In the mold-formed paper substrate manufacturing method 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. As a result, dewatering in the papermaking process is improved, and operability is also improved.

[0054] Conventionally, paper substrates have been manufactured using cylinder-type paper machines. However, cylinder-type paper machines have insufficient flexibility in the longitudinal direction. In contrast, by manufacturing paper using a short-type paper machine as in the present invention, the tensile strength-to-width ratio becomes smaller (less than 2) than when using a cylinder-type paper machine. As a result, the press pressure during mold forming is evenly distributed in both the longitudinal and transverse directions, reducing the occurrence of localized loads. Consequently, cracking, tearing, and wrinkles caused by surface layer displacement can be suppressed. In addition, the manufacturing method of the present invention employs multi-layer papermaking rather than single-layer papermaking. Therefore, inter-layer displacement is mitigated in response to the shear stress applied in the thickness direction of the paper during deep drawing. As a result, the mold-molded paper base material can more easily follow the mold used for deep drawing during the molding process. Therefore, according to the manufacturing method of the present invention, a die-molded paper substrate with excellent processability is obtained when obtaining a paper molded body with sufficient strength for practical use.

[0055] Since mold-formed paper substrates are obtained by lamination, their thickness can be freely adjusted. Therefore, when multiple mold-formed paper substrates are glued together, the number of sheets to be glued can be easily reduced. As a result, a reduction in the amount of glue used and a reduction in drying time can be expected.

[0056] <Applications of mold-molded paper substrates> The mold-molded paper substrate can be suitably used in mold molding to obtain various paper molded products. In mold molding, it is preferable to position the second surface layer on the convex side during mold molding. This is because the second surface layer, which contains pulp with relatively high freeness, is more easily plastically deformed and has superior flexibility than the first surface layer.

[0057] The paper molded products made from mold-molded paper substrates do not have any particular applications. Preferred examples include, for instance, deep-drawn paper plates, paper cutlery, and paper hangers. Other examples include relatively deep paper trays and paper cups.

[0058] Although several embodiments have been described above, the present invention is not limited to the embodiments disclosed herein and can be implemented with appropriate modifications without altering the spirit of the invention. The embodiments disclosed herein can be implemented in various other forms, and various omissions, substitutions, and modifications are possible without departing from the spirit of the invention. [Examples]

[0059] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the following description.

[0060] <Example 1> (Preparation of pulp slurry for the first surface layer) For the first surface layer, a pulp slurry of 100% LBKP was obtained by beating Freeness to 325 ml. To the obtained pulp slurry, 0.7% by mass of a paper strength enhancer (PAM-based paper strength enhancer), 0.1% by mass of cationized starch, 0.1% by mass of a rosin-based internal sizing agent, and 1.0% of aluminum sulfate as a fixative were added, each per 100% by mass of the dry mass of the pulp, to prepare the pulp slurry for the first surface layer.

[0061] (Preparation of pulp slurry for the second surface layer) For the second surface layer, a pulp slurry of 100% LBKP was obtained by beating Freeness to 456 ml. To the obtained pulp slurry, 0.7% by mass of a paper strength enhancer (PAM-based paper strength enhancer), 0.1% by mass of cationized starch, 0.3% by mass of a rosin-based internal sizing agent, and 1.3% of aluminum sulfate as a fixative were added, each per 100% by mass of the dry mass of the pulp, to prepare the pulp slurry for the second surface layer.

[0062] (Preparation of pulp slurry for the first subsurface paper layer) As the pulp component to be used for the first subsurface papermaking layer, a pulp slurry of 100% LBKP was obtained by beating Freeness to 366 ml. To the obtained pulp slurry, 0.4% by mass of a paper strength enhancer (PAM-based paper strength enhancer), 0.1% by mass of cationized starch, 0.4% by mass of a rosin-based internal sizing agent, and 1.0% of aluminum sulfate as a fixative were added, each per 100% by mass of the dry mass of the pulp, to prepare the pulp slurry for the first subsurface papermaking layer.

[0063] (Preparation of pulp slurry for the papermaking layer below the second surface) As the pulp component to be used for the second subsurface papermaking layer, a pulp slurry of 100% LBKP was obtained by beating Freeness to 430 ml. To the obtained pulp slurry, 0.5% by mass of a paper strength enhancer (PAM-based paper strength enhancer), 0.1% by mass of cationized starch, 0.4% by mass of a rosin-based internal sizing agent, and 1.2% of aluminum sulfate as a fixative were added, each per 100% by mass of the dry mass of the pulp, to prepare the pulp slurry for the second subsurface papermaking layer.

[0064] (Preparation of the pulp slurry for the first intermediate layer) For the first intermediate layer, a pulp slurry of 100% LBKP was obtained by beating Freeness to 385 ml. To the obtained pulp slurry, 0.4% by mass of a paper strength enhancer (PAM-based paper strength enhancer), 0.4% by mass of a rosin-based internal sizing agent, and 1.1% of aluminum sulfate as a fixative were added, each per 100% by mass of the dry mass of the pulp, to prepare the pulp slurry for the first intermediate layer.

[0065] (Preparation of pulp slurry for the second intermediate layer) For the second intermediate layer, a pulp slurry of 100% LBKP was obtained by beating Freeness to 393 ml. To the obtained pulp slurry, 0.4% by mass of a paper strength enhancer (PAM-based paper strength enhancer), 0.4% by mass of a rosin-based internal sizing agent, and 1.1% of aluminum sulfate as a fixative were added, each per 100% by mass of the dry mass of the pulp, to prepare the pulp slurry for the second intermediate layer.

[0066] (Papermaking) After forming each prepared pulp slurry on a Fourdrinier multi-cylinder paper machine and then drying each paper layer, a mold forming paper substrate with a basis weight of 751 g / m² and a paper thickness of 913 μm having a 6-layer structure was obtained. 2 A mold forming paper substrate having a basis weight of 751 g / m² and a paper thickness of 913 μm having a 6-layer structure was obtained.

[0067] <Example 2> By the same operations and methods as in Example 1, except that the aspect ratio of the tensile strength in Example 1 was changed to 1.42, a mold forming paper substrate with a basis weight of 708 g / m² and a paper thickness of 952 μm was obtained. 2 A mold forming paper substrate having a basis weight of 708 g / m² and a paper thickness of 952 μm was obtained.

[0068] <Example 3> The same operations and methods as in Example 1 were carried out, and further, a smoothing treatment was performed by a machine calendar, and a mold forming paper substrate with a basis weight of 635 g / m² and a paper thickness of 756 μm was obtained. 2 A mold forming paper substrate having a basis weight of 635 g / m² and a paper thickness of 756 μm was obtained.

[0069] <Comparative Example 1> A mold forming paper substrate was obtained by disposing the second surface paper layer of the mold forming paper substrate A in Example 1 on the convex side during mold forming.

[0070] <Comparative Example 2> In Example 3, as the pulp used for the first surface layer and the second surface layer, the pulp was beaten to a freeness of 450 ml, and a pulp slurry with a pulp composition of 39% NBKP and 61% LBKP was obtained. To the obtained pulp slurry, 0.9% by mass of a paper strength enhancer (PAM-based paper strength enhancer), 0.6% by mass of a rosin-based internal sizing agent, and 1.9% of aluminum sulfate as a fixing agent were added respectively with respect to 100% by mass of the dry mass of the pulp, and pulp slurries for the first surface layer and the second surface layer were prepared.

[0071] For the first subsurface paper layer, the first intermediate paper layer, the second intermediate paper layer, and the second subsurface paper layer, Freeness was beaten to 420 ml to obtain a pulp slurry with a pulp composition of 30% NBKP and 70% LBKP. To the obtained pulp slurry, 0.5% by mass of a paper strength enhancer (PAM-type paper strength enhancer), 0.5% by mass of a rosin-type internal sizing agent, and 1.5% of aluminum sulfate as a fixative were added, each per 100% by mass of the dry mass of the pulp, to prepare pulp slurries for the first subsurface paper layer, the first intermediate paper layer, the second intermediate paper layer, and the second subsurface paper layer.

[0072] Except for the preparation of the pulp slurry for each layer, the same procedure and method as in Example 3 was used to obtain a basis weight of 412 g / m². 2 A mold-molded paper substrate with a paper thickness of 470 μm was obtained.

[0073] <Comparative Example 3> In Example 3, the short-wire multi-cylinder paper machine was changed to a round-wire multi-cylinder paper machine, and a five-layer structure was adopted with the first surface paper layer, first subsurface paper layer, middle paper layer, second subsurface paper layer, and second surface paper layer in that order. Except for changing the basis weight, paper thickness, and pulp slurry freeness of each layer according to Table 1, the procedure was the same as in Example 3, resulting in a basis weight of 416 g / m². 2 A mold-molded paper substrate with a paper thickness of 426 μm was obtained.

[0074] <Comparative Example 4> Using a cylindrical multi-cylinder paper machine, a six-layer structure was created, consisting of 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.

[0075] For the first and second surface layers, Freeness was beaten to 385 ml to obtain a pulp slurry with a pulp composition of 13% NBKP and 87% LBKP. To the obtained pulp slurry, 3.0% by mass of a paper strength enhancer (PAM-based paper strength enhancer) was added per 100% by mass of the dry mass of the pulp to prepare the pulp slurries for the first and second surface layers.

[0076] For the first subsurface paper layer, the first intermediate paper layer, the second intermediate paper layer, and the second subsurface paper layer, Freeness was beaten to 380 ml to obtain a pulp slurry with a pulp composition of 37% NBKP and 63% LBKP. To the obtained pulp slurry, 3.0% by mass of a paper strength enhancer (PAM-type paper strength enhancer) and 1.5% by mass of an alkyl ketene dimer-type internal sizing agent were added, each per 100% by mass of the dry mass of the pulp, to prepare pulp slurries for the first subsurface paper layer, the first intermediate paper layer, the second intermediate paper layer, and the second subsurface paper layer.

[0077] (Papermaking) After each prepared pulp slurry is made into paper using a cylinder-type multi-cylinder paper machine, and then each paper layer is dried, a 6-layer structure with a basis weight of 416 g / m² is produced. 2 A mold-molded paper substrate with a paper thickness of 480 μm was obtained.

[0078] <Measurement method> For each example of mold-molded paper substrate, various physical properties were measured as described below.

[0079] (Freeness of each layer) The freeness of each layer of the base paper was measured using a Canadian standard water filtration tester in accordance with JIS P 8121-2:2012, after the sample was dissociated using a standard dissociator in accordance with JIS P 8220-2:2012.

[0080] (Total basis weight of mold-molded paper substrate) The basis weight of the base paper was measured in accordance with JIS P 8124:2011.

[0081] (Total thickness of mold-molded paper substrate) The paper thickness of the base paper was measured in accordance with JIS P 8118:2014.

[0082] (Overall density of mold-molded paper substrate) The density of the base paper was calculated from the basis weight and paper thickness mentioned above.

[0083] (Ratio of tensile strength to aspect ratio) The tensile strength in the longitudinal and transverse directions of the base paper was measured in accordance with JIS P 8113:2006, and the ratio of the longitudinal tensile strength to the transverse tensile strength was calculated from the obtained measurements.

[0084] (Tensile elongation at fracture) The tensile elongation at break of the base paper was measured in accordance with JIS P 8113:2006.

[0085] (Taber stiffness) The Taber stiffness of the base paper was determined in accordance with JIS P 8125:2000, and the longitudinal value was calculated relative to the transverse value of the obtained Taber stiffness.

[0086] (Overall interlayer strength of mold-molded paper substrate) The interlayer strength of the base paper was measured in accordance with JIS P 8131:2009.

[0087] (Oken type smoothness) The smoothness of the base paper was measured according to the Ogura method in accordance with JIS P 8155:2010.

[0088] (Degree of bump size of mold-molded paper substrate) The knot size of the base paper was measured in accordance with JIS P 8140:1998.

[0089] <Evaluation Method> Each example of mold-molded paper substrate was conditioned for 72 hours in an environment of 30 degrees Celsius and 90% RH, then cut to B5 size, and the four corners were cut into 5 cm radius arcs to create blanks. A bento box-shaped mold was used for the blanks. 1 By placing the surface layer side towards the female mold and performing deep drawing, a paper molded body with a depth of 24 mm, a long side of 264 mm, and a short side of 113 mm was obtained.

[0090] The processability of the obtained paper molded articles for deep drawing was evaluated as described below. A result of ○ or △ was considered a pass. The evaluation results are shown in Table 1. (Processing suitability during deep drawing: length of cracks at corners) The length of cracks in the rounded corners of the paper molded body was evaluated according to the following criteria. ○: No cracks are visible in the rounded corners. △: The length of the crack in the rounded corner is 10 mm or less. ×: The length of the crack in the rounded corner exceeds 10 mm.

[0091] (Processing suitability during deep drawing: width of cracks at corners) ○: No cracks are visible in the rounded corners. △: There is a crack in the rounded corner, but it is linear and not a complete tear. ×: The rounded corner is torn or appears to be ripped.

[0092] [Table 1]

[0093] Examples 1 to 3 yielded mold-molded paper substrates with excellent processability for obtaining paper molded articles with sufficient strength for practical use. In contrast, Comparative Examples 1 to 4 failed to achieve both sufficient strength for practical use and processability during deep drawing. [Industrial applicability]

[0094] According to the present invention, a mold-molded paper substrate with excellent processability for obtaining a paper molded article having sufficient strength for practical use is provided, as well as a method for manufacturing the same.

Claims

1. A mold-molded 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 in the first surface layer is lower than the freeness of the pulp in the middle layer. The freeness of the pulp in the second surface layer is higher than the freeness of the pulp in the middle layer. The aspect ratio of the tensile strength is less than 2. A mold-molding paper substrate used by positioning the second surface layer on the convex side during mold molding.

2. The mold-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.

3. A method for manufacturing a mold-molded paper substrate having at least a first surface layer, a middle layer, and a second surface layer in this order, wherein the second surface layer is positioned on the convex side during mold molding, A first papermaking layer having at least a first surface papermaking layer obtained by papermaking a first surface layer pulp slurry containing pulp with lower freeness than the middle layer pulp using a short-wire multi-cylinder papermaking machine, A second papermaking layer having at least a second surface papermaking layer obtained by papermaking a second surface layer pulp slurry containing pulp with higher freeness than the aforementioned middle layer pulp using a short-wire multi-cylinder papermaking machine, A manufacturing method characterized by combining two materials.

4. The manufacturing method according to claim 3, wherein the freeness of the first surface layer pulp slurry is lower than the freeness of the second surface layer pulp slurry.

Images