Transparent paper and method for manufacturing transparent paper

Abstract

To provide a transparent paper and a method for producing the transparent paper, which maintains sufficient transparency as a transparent paper and has excellent heat resistance and water resistance.SOLUTION: The present invention relates to a transparent paper 1 in which a pulp-containing base material 10 is impregnated with a clarifying agent, wherein the clarifying agent is a cured product of a composition containing (a) an ester having two or more hydroxyl groups and (b) a melamine compound or isocyanate compound having at least one group selected from the group consisting of a methylol group, a methoxy group and an isocyanate group, wherein the mass ratio (b / a) of the (b) component to the (a) component is 15 mass% or more, and wherein the ratio of the clarifying agent to the basis weight of the base material 10 is 10% by mass or more. Thus the transparent paper 1 of the invention is provided.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to transparencies and methods for making transparencies. [Background technology]

[0002] Transparent paper and translucent paper (hereinafter collectively referred to as "transparent paper") are suitable for use in applications requiring a certain degree of visibility, such as food packaging, medicine packaging, window envelopes, etc. They are also used as release paper for adhesive sheets, etc.

[0003] As an example of such transparent paper, Patent Document 1 discloses transparent paper containing at least pulp and wax having a melting point of 35°C or higher under normal pressure, the pulp having a Canadian standard freeness of 300 mL or more and 600 mL or less, at least one type of pulp being LBKP, and the LBKP content being 70 mass% or more of the total pulp amount.

[0004] Furthermore, Patent Document 2 discloses a transparent paper characterized in that the base paper is impregnated with an acrylate having eight or more ethylene oxide chains and cured by irradiation with radiation. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2020-029626 [Patent Document 2] Japanese Patent Application Publication No. 03-146798 Summary of the Invention [Problem to be solved by the invention]

[0006] However, conventional transparent papers, including those disclosed in Patent Documents 1 and 2, still have room for improvement in terms of achieving high levels of heat resistance and water resistance while maintaining sufficient transparency as transparent paper. For example, the transparent paper disclosed in Patent Document 1 has a problem of wax migration. Therefore, there is a demand for transparent paper that maintains a certain level of transparency while improving the balance of heat resistance and water resistance.

[0007] The present invention has been made in consideration of the above-mentioned circumstances, and aims to provide transparent paper and a method for manufacturing transparent paper that has excellent heat resistance and water resistance while maintaining sufficient transparency as transparent paper. [Means for solving the problem]

[0008] As a result of intensive research into achieving the above-mentioned object, the present inventors have discovered and completed the present invention, which provides transparent paper in which a pulp-containing substrate is impregnated with a clarifying agent, the clarifying agent being a cured product of a composition containing (a) an ester having two or more hydroxyl groups, and (b) a melamine compound or an isocyanate compound having at least one group selected from the group consisting of a methylol group, a methoxy group, and an isocyanate group, the mass ratio (b / a) of component (b) to component (a) being 15 mass% or more, and the proportion of the clarifying agent to the basis weight of the substrate being 10 mass% or more.

[0009] That is, the present invention is as follows.

[0010] (1) The transparent paper is obtained by impregnating a pulp-containing substrate with a clarifying agent, wherein the clarifying agent is a cured product of a composition containing (a) an ester having two or more hydroxyl groups, and (b) a melamine compound or an isocyanate compound having at least one group selected from the group consisting of a methylol group, a methoxy group, and an isocyanate group, wherein the mass ratio (b / a) of the component (b) to the component (a) is 15 mass% or more, and the proportion of the clarifying agent to the basis weight of the substrate is 10 mass% or more. (2) The transparent paper according to (1), wherein the melamine compound is an alkylated melamine. (3) The transparent paper according to (1) or (2) further comprises a barrier layer on at least one surface of the substrate. (4) The transparent paper according to any one of (1) to (3) above has an opacity of 60% or less in accordance with JIS P8149. (5) A method for producing transparent paper in which a pulp-containing substrate is impregnated with a clarifying agent, the method comprising the steps of: impregnating a pulp-containing substrate with a clarifying agent composition; and drying the substrate, wherein the clarifying agent composition comprises: (a) an ester having two or more hydroxyl groups; (b) a melamine compound or an isocyanate compound having at least one group selected from the group consisting of a methylol group, a methoxy group, and an isocyanate group; and (c) an organic solvent, wherein the mass ratio (b / a) of the component (b) to the component (a) is 15 mass% or more; and the proportion of the clarifying agent to the basis weight of the substrate is 10 mass% or more. (6) The method for producing transparent paper according to (5), wherein the solid content concentration of the clarifying agent composition is 35% by mass or more and 70% by mass or less. (7) The method for producing transparent paper according to (5) or (6), wherein the viscosity of the clarifying agent composition is 40 mPa·s or less. (8) The method for producing transparent paper according to any one of (5) to (7), wherein the melamine compound is an alkylated melamine. (9) The method for producing transparent paper according to any one of (5) to (8), wherein the step of impregnating the substrate with the clarifying agent composition is a step of coating the substrate with the clarifying agent composition, and the substrate has a barrier layer on a surface opposite to the surface coated with the clarifying agent composition. (10) The method for producing transparent paper according to any one of (5) to (9), wherein the opacity of the transparent paper according to JIS P8149 is 60% or less. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide transparent paper and a method for manufacturing transparent paper that has excellent heat resistance and water resistance while maintaining sufficient transparency as transparent paper. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic cross-sectional view of a transparent paper sheet according to the first embodiment. [Figure 2] FIG. 2 is a schematic cross-sectional view of a transparent paper sheet according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, a mode for carrying out the present invention (hereinafter simply referred to as "the present embodiment") will be described in detail. The following present embodiment is an example for explaining the present invention, and is not intended to limit the present invention to the following content. The present invention can be carried out by appropriately modifying it within the scope of its gist.

[0014] In the drawings, the same elements are given the same reference numerals, and redundant explanations will be omitted. Furthermore, unless otherwise specified, the positional relationships such as up, down, left, and right are based on the positional relationships shown in the drawings. Furthermore, the dimensional ratios of the drawings are not limited to the ratios shown in the drawings.

[0015] In addition, in this specification, unless otherwise specified, "(meth)acrylic" includes methacryl and acrylic. For example, (meth)acrylic means methacryl, acrylic, or both.

[0016] <Transparent paper>

[0017] FIG. 1 is a schematic cross-sectional view of a transparent paper sheet according to the first embodiment.

[0018] The transparent paper 1 according to this embodiment is a transparent paper 1 in which a pulp-containing substrate 10 is impregnated with a clarifying agent, the clarifying agent being a cured product of a composition containing (a) an ester having two or more hydroxyl groups and (b) a melamine compound or an isocyanate compound having at least one group selected from the group consisting of a methylol group, a methoxy group, and an isocyanate group, the mass ratio (b / a) of component (b) to component (a) being 15% by mass or more, and the proportion of the clarifying agent relative to the basis weight of the substrate 10 being 10% by mass or more. Note that the term "transparent paper" in this specification refers to thin, transparent or translucent paper, and includes paper generally referred to as "transparent paper" and paper generally referred to as "semitransparent paper."

[0019] According to this embodiment, it is possible to provide the transparent paper 1 with excellent heat resistance and water resistance while maintaining sufficient transparency. The reason for this is not clear, but is presumed to be as follows.

[0020] First, components (a) and (b) penetrate into the interior of substrate 10 and undergo a curing reaction, so that the cured product can exist in a state impregnated inside substrate 10. This cured product acts as a clarifying agent and effectively fills the voids inside substrate 10. It is believed that if there are no voids inside substrate 10, light scattering will not occur, and therefore sufficient transparency can be achieved.

[0021] Furthermore, because components (a) and (b) cure via a crosslinking reaction inside the substrate 10, migration of the clarifying agent and seepage to the surface can be effectively suppressed, which is believed to provide excellent heat resistance and water resistance. For example, even if the transparent paper 1 is stored or left standing for a long period of time in a high-temperature environment such as a car dashboard in the summer, the clarifying agent component of the transparent paper 1 can be prevented from thermally melting and seeping out to the paper surface (however, the effects of this embodiment are not limited to these).

[0022] (base material)

[0023] The upper limit of the basis weight of the substrate 10 is 100 g / m 2Preferably less than 80 g / m 2 More preferably, it is 70 g / m or less. 2 More preferably, it is 65 g / m or less. 2 Furthermore, the lower limit of the basis weight is 30 g / m or less from the viewpoint of strength. 2 It is preferable that the weight is 40 g / m or more. 2 More preferably, it is 50 g / m or more. 2 More preferably, it is 60 g / m or more. 2 By setting the basis weight within the above range, the components (a) and (b) can penetrate deep into the substrate 10, thereby imparting sufficient transparency.

[0024] The upper limit of the thickness of the substrate 10 is preferably 100 μm or less, more preferably 65 μm or less, from the viewpoint of transparency, and the lower limit of the thickness is preferably 30 μm or more, more preferably 50 μm or more, from the viewpoint of strength.

[0025] The substrate 10 may be made of any material that contains at least pulp, and may be selected appropriately depending on the intended use. Examples of the substrate 10 include bleached or unbleached kraft paper (acid paper or neutral paper), fine paper, medium-quality paper, lightly coated paper, coated paper, liner, semi-glassine paper, glassine paper, one-side glazed paper, and white cardboard.

[0026] Conventionally, a method for ensuring transparency as transparent paper has been to use a paper substrate that has been acidified with sulfuric acid or the like, such as parchment paper. However, according to the present embodiment, sufficient transparency can be imparted without using such an acid-treated paper substrate. That is, one preferred aspect of the present embodiment is to use a substrate 10 that has not been acid-treated.

[0027] Another conventional method for ensuring transparency as transparent paper involves using a paper substrate made from highly beaten pulp. For example, a papermaking method may be used in which paper is made from highly beaten pulp raw material, followed by a strong supercalendering process to bond the fibers together and thereby remove as much air as possible from the pulp fibers and the sheet. Transparent paper that has undergone such a highly beaten process has a pulp beating degree ranging from approximately 70°SR to 90°SR. However, this method tends to result in insufficient dehydration on the papermaking machine, resulting in poor productivity and limited transparency. In this regard, the present embodiment can provide sufficient transparency without using a highly beaten paper substrate. That is, the substrate 10 used in this embodiment may be beaten, but it also has the advantage that beating is not necessarily required. Furthermore, since the beating process can be omitted or simplified, this is a simple manufacturing method, and it can also contribute to reducing beating power, i.e., the unit power consumption, and therefore is expected to be advantageous from the perspective of manufacturing costs. That is, in one preferred aspect of this embodiment, the base material 10 is made primarily of pulp, and the pulp used has a beating degree of 60°SR or less, based on the Shopper-Rigler method, preferably 40°SR or less, and more preferably 30°SR or less.

[0028] (clarifying agent)

[0029] The clarifying agent is impregnated into the substrate 10 and is a cured product of a composition containing (a) an ester having two or more hydroxyl groups, and (b) a melamine compound or an isocyanate compound having at least one group selected from the group consisting of a methylol group, a methoxy group, and an isocyanate group. As described above, in this embodiment, the clarifying agent can be present in the substrate 10 as a cured product by impregnating the substrate 10 with the components (a) and (b) and curing them.

[0030] Conventionally, clarifying agents have been used, such as oil plasticizers, liquid paraffin, polystyrene resins, and water-based polymer materials such as styrene copolymers, as well as high-melting point compounds. Attempts have been made to improve the transparency of transparent paper by applying such clarifying agents to the surface of the paper substrate. However, this method has the problem that, during long-term storage of the transparent paper, external heat can cause the clarifying agent components to migrate and bleed onto the surface of the transparent paper. Furthermore, conventional water-based clarifying agents tend to produce coating solutions with high viscosity, which can lead to poor application of the clarifying agent, resulting in the coating solution remaining on the surface of the paper substrate and making it difficult to penetrate into the interior of the paper substrate. This also creates the problem of the clarifying agent not being able to fully fill the voids within the paper substrate, resulting in insufficient improvement in transparency.

[0031] In this regard, according to the present embodiment, a liquid clarifying agent composition containing component (a), component (b), and a solvent can be used as a coating liquid, allowing these to be impregnated deeper into the substrate 10 and to penetrate into voids present within the substrate 10. Then, by the curing reaction of component (a) and component (b), these cured products effectively fill voids within the paper in the substrate as a clarifying agent. Therefore, problems that can occur in the above-mentioned conventional methods, such as migration of the clarifying agent components and insufficient improvement in transparency, can be effectively suppressed.

[0032] Component (a) is an ester having two or more hydroxyl groups. Specific examples of component (a) include sorbitan fatty acid esters, glycerin fatty acid esters, pentaerythritol fatty acid esters, and sucrose fatty acid esters.

[0033] Examples of sorbitan fatty acid esters that can be used include esters in which a fatty acid is bonded to one or two of the four hydroxyl groups of sorbitan (such as sorbitan fatty acid monoesters and sorbitan fatty acid diesters), and esters in which a fatty acid is bonded to a hydroxyl group of polyoxyalkylene sorbitan. Specific examples of sorbitan fatty acid esters include sorbitan monooleate, sorbitan monostearate, sorbitan monomyristate, sorbitan monolaurate, sorbitan dioleate, sorbitan distearate, sorbitan dimyristate, and sorbitan dilaurate; polyoxyalkylene sorbitan monooleates (such as polyoxyethylene sorbitan monooleate and polyoxypropylene sorbitan monooleate); ... Examples of the polyoxyalkylene sorbitan fatty acids include polyoxyalkylene sorbitan monostearate, polyoxyalkylene sorbitan monomyristate, polyoxyalkylene sorbitan monolaurate, polyoxyalkylene sorbitan dioleate (e.g., polyoxyethylene sorbitan dioleate, polyoxypropylene sorbitan dioleate), polyoxyalkylene sorbitan distearate, polyoxyalkylene sorbitan dimyristate, and polyoxyalkylene sorbitan dilaurate.

[0034] The polyoxyalkylene sorbitan fatty acid that can be used includes those in which the polyoxyalkylene group is a polyoxyethylene group having 1 to 20 (preferably 2 to 10, more preferably 3 to 9) ethylene oxide units, or a polyoxypropylene group having 1 to 20 (preferably 2 to 10, more preferably 3 to 9) propylene oxide units.

[0035] Examples of glycerin fatty acid esters that can be used include those in which a fatty acid is ester-bonded to one of the three hydroxyl groups of glycerin, and those in which a fatty acid is ester-bonded to a hydroxyl group of polyglycerin. Specific examples of glycerin fatty acid esters include glycerin fatty acid esters such as glycerin monooleate, glycerin monostearate, glycerin monomyristate, glycerin monolaurate, glycerin dioleate, glycerin distearate, glycerin dimyristate, and glycerin dilaurate; polyglycerin esterified with oleic acid; polyglycerin esterified with stearic acid; polyglycerin esterified with myristic acid; and polyglycerin esterified with lauric acid.

[0036] Examples of sucrose fatty acid esters that can be used include esters in which a fatty acid is bound to a hydroxyl group of sucrose, etc. Specific examples of sucrose fatty acid esters include sucrose oleate, sucrose behenate, sucrose stearate, sucrose palmitate, sucrose myristate, sucrose laurate, and sucrose erucate.

[0037] Examples of pentaerythritol fatty acid esters include pentaerythritol monooleate, pentaerythritol monostearate, pentaerythritol monomyristate, pentaerythritol monolaurate, pentaerythritol dioleate, pentaerythritol distearate, pentaerythritol dimyristate, and pentaerythritol dilaurate.

[0038] Component (b) is a melamine compound or an isocyanate compound having at least one group selected from the group consisting of a methylol group, a methoxy group, and an isocyanate group. Component (b) only needs to have these groups as reactive groups capable of crosslinking with component (a), and components (a) and (b) can exist as a cured product in substrate 10.

[0039] Specific examples of the melamine compound include at least one selected from the group consisting of melamine compounds such as methylol melamine, alkylated melamine (methylated melamine, ethylated melamine, propylated melamine, butylated melamine, etc.), methoxy melamine, imino-type melamine, imino-methylol-type melamine, etc., and resins (melamine resins) containing structural units derived from any one of these. These melamine compounds and melamine resins may be used alone or in combination of two or more.

[0040] Examples of the isocyanate compound include aliphatic polyisocyanates, alicyclic polyisocyanates, and aromatic polyisocyanates, each of which has two or more (preferably two or more and four or less, more preferably two or more and three or less) isocyanate groups in the molecule, from the viewpoint of curability with component (a).

[0041] Examples of the aliphatic polyisocyanate include ethylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, pentane diisocyanate, and lysine diisocyanate.

[0042] Examples of alicyclic polyisocyanates include isophorone diisocyanate, cyclohexyl diisocyanate, cyclopentyl diisocyanate, hydrogenated xylylene diisocyanate, hydrogenated tolylene diisocyanate, hydrogenated diphenylmethane diisocyanate, hydrogenated tetramethylxylene diisocyanate, and 4,4'-dicyclohexylmethane diisocyanate.

[0043] Examples of aromatic polyisocyanates include tolylene diisocyanate, diphenylmethane diisocyanate, diphenyl ether diisocyanate, nitrodiphenyl diisocyanate, diphenylpropane diisocyanate, dimethyldiphenylmethane diisocyanate, diphenylpropane diisocyanate, phenylene diisocyanate, naphthylene diisocyanate, dimethoxydiphenyl diisocyanate, and xylylene diisocyanate.

[0044] Furthermore, it may be a multimer (dimer, trimer, etc.) or derivative of a bifunctional or trifunctional or higher functional isocyanate (for example, an addition reaction product of a polyhydric alcohol and two or more molecules of a polyfunctional isocyanate).

[0045] These may be used alone or in combination of two or more.

[0046] Examples of solvents that can be used together with component (a) and component (b) during production include aromatic hydrocarbons such as toluene and benzene; aliphatic hydrocarbons such as heptane, hexane, pentane, and isooctane; alcohols such as ethanol and methanol; ketones such as acetone and methyl ethyl ketone; esters such as methyl acetate, ethyl acetate, and butyl acetate; ethers such as tetrahydrofuran (THF); diethyl ether (DME), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and carbon tetrachloride. These may be used alone or in combination of two or more.

[0047] The lower limit of the mass ratio (b / a) of component (b) to component (a) is 15% by mass or more, preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 35% by mass or more, from the viewpoint of sufficiently maintaining the curability of components (a) and (b) and thereby maintaining heat resistance, water resistance, etc. The upper limit of the mass ratio (b / a) is preferably 70% by mass or less, more preferably 60% by mass or less, even more preferably 50% by mass or less, and even more preferably 45% by mass or less.

[0048] The lower limit of the ratio of the clarifying agent to the basis weight of the substrate 10 ("clarifying agent ratio," mass %) is 10% by mass or more, preferably 12% by mass or more, more preferably 15% by mass or more, and even more preferably 18% by mass or more, from the viewpoint of transparency. The upper limit of the clarifying agent ratio is preferably 40% by mass or less, more preferably 30% by mass or less, and even more preferably 25% by mass or less, from the viewpoint of production costs and imparting transparency. The ratio of the clarifying agent to the basis weight of the substrate 10 ("clarifying agent ratio") can be adjusted, for example, by selecting the cell volume of the Mayer bar.

[0049] The ratio of the clarifying agent to the basis weight of the substrate 10 ("ratio of the clarifying agent", mass %) can be calculated according to the following formula 1. Percentage of clarifying agent (mass%) = Dry adhesion amount of clarifying agent (g / m 2 ) / Basis weight of substrate (g / m 2 )×100...(Formula 1) The “dry adhesion amount of the clarifying agent” in formula 1 refers to the dry adhesion amount of the clarifying agent per unit area of ​​the substrate 10.

[0050] FIG. 2 is a schematic cross-sectional view of a transparent paper sheet according to the second embodiment.

[0051] The second embodiment differs from the first embodiment in that a barrier layer 20 is provided on the transparent paper 1. As described above, in this embodiment, it is preferable that the substrate 10 further has a barrier layer 20 on at least one surface. The barrier layer 20 is preferably provided on only one surface (one side) of the substrate 10. When producing this embodiment, a liquid clarifying agent composition containing components (a) and (b) is applied to the surface of the substrate 10, impregnated into the interior thereof, and then cured, thereby disposing the clarifying agent inside the substrate 10. When employing such a method, by providing the barrier layer 20 on only one surface, the transparentizing agent composition can be applied to the substrate 10 from the surface opposite the barrier layer 20, thereby effectively preventing penetration and leakage of the transparentizing agent composition from the surface on the barrier layer 20 side.

[0052] To further explain the advantage of providing the barrier layer 20, by disposing the barrier layer 20 on the surface (back surface) opposite to the coated surface of the substrate 10, not only can the transparentizing agent composition be prevented from permeating to the back surface and leaking, but also contamination of members such as guide rolls in a coating machine can be prevented, thereby enabling mass production using a roll-support type coating machine or the like.

[0053] The barrier layer 20 can be made of a material that can be used as a sealing layer, etc. Examples include polyvinyl alcohol (PVA), styrene butadiene rubber, starch, polyethylene, silicone, fluorine, etc. The sealing layer may contain fillers such as kaolin, talc, titanium, silica, etc., as well as binders.

[0054] The upper limit of the opacity of the transparent papers 1 and 2 according to JIS P8149 is preferably 60% or less, more preferably 55% or less, and even more preferably 50% or less. The lower limit of the opacity is preferably 3% or more, more preferably 5% or more, even more preferably 10% or more, even more preferably 40% or more, and even more preferably 45% or more.

[0055] <Manufacturing method>

[0056] A suitable example of the production method according to the present embodiment is a method for producing transparent paper 1 in which a pulp-containing substrate 10 is impregnated with a clarifying agent, the method including the steps of impregnating the pulp-containing substrate 10 with a clarifying agent composition and drying the substrate 10, wherein the clarifying agent composition includes (a) an ester having two or more hydroxyl groups, (b) a melamine compound or an isocyanate compound having at least one group selected from the group consisting of a methylol group, a methoxy group, and an isocyanate group, and (c) an organic solvent, wherein the mass ratio (b / a) of the component (b) to the component (a) is 15 mass% or more, and the proportion of the clarifying agent relative to the basis weight of the substrate 10 is 10 mass% or more.

[0057] In the production method according to this embodiment, the lower limit of the mass ratio (b / a) of component (b) to component (a) is 15% by mass or more, preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 35% by mass or more, from the viewpoint of sufficiently maintaining the curability of components (a) and (b) and thereby maintaining heat resistance, water resistance, etc. The upper limit of the mass ratio (b / a) is preferably 70% by mass or less, more preferably 60% by mass or less, even more preferably 50% by mass or less, and even more preferably 45% by mass or less.

[0058] The clarifying agent composition contains the above-mentioned components (a) and (b) in an organic solvent (c). Any organic solvent capable of dissolving components (a) and (b) can be used. Specific examples of organic solvents include aromatic hydrocarbons such as toluene and benzene; acetate esters such as ethyl acetate; aliphatic hydrocarbons such as hexane; alicyclic hydrocarbons such as cyclohexane; halogenated alkanes such as 1,2-dichloroethane; alcohols such as methanol, ethanol, and isopropyl alcohol; ethers such as tert-butyl methyl ether; and ketones such as methyl ethyl ketone. These may be used alone or in combination of two or more. The clarifying agent composition is preferably non-aqueous, and more preferably does not contain water.

[0059] The clarifying agent composition may further contain a catalyst (d). The inclusion of a catalyst is preferred because it can accelerate the curing of components (a) and (b). The catalyst (d) may be, for example, at least one selected from the group consisting of acid catalysts such as p-toluenesulfonic acid and sulfuric acid; alkyl acetate catalysts such as methyl acetoacetate and ethyl acetoacetate; amines; and zirconium.

[0060] The clarifying agent composition may further contain, as necessary, a filler, a plasticizer, an ultraviolet absorber, an antioxidant, an antiaging agent, a water repellent (such as wax), an oil-resistant agent (such as fluororesin), and the like.

[0061] The solid content concentration of the clarifying agent composition is preferably 35% by mass or more and 70% by mass or less. The lower limit of the solid content concentration is more preferably 40% by mass or more, and even more preferably 55% by mass or more. The solid content concentration here refers to the proportion of non-volatile components in the clarifying agent composition. The non-volatile components refer to components that do not volatilize under conditions of 50°C for 3 hours. The solid content concentration can be calculated from the mass of the composition after heating at 50°C for 3 hours and the mass before heating.

[0062] The viscosity of the clarifying agent composition is preferably 40 mPa·s or less. The lower limit of the viscosity is preferably 10 mPa·s or more, and more preferably 13 mPa·s or more. By setting the viscosity within the above range, the clarifying agent composition can be easily impregnated into the substrate 10, and as a result, the clarifying agent composition can be cured inside the substrate 10, thereby further improving transparency.

[0063] The method for impregnating the clarifying agent composition into the substrate 10 is not particularly limited, and any known method can be used. For example, the impregnation can be performed using a coating machine such as a bar coater, a die coater, a gravure coater, a roll coater, a blade coater, an air knife coater, or a size press coater.

[0064] When the substrate 10 has a barrier layer 20, such as the transparent paper 2, the step of impregnating the substrate 10 with the clarifying agent composition is a step of coating the substrate 10 with the clarifying agent composition, and the substrate 10 preferably has a barrier layer 20 on the surface opposite to the surface to which the clarifying agent composition is coated. In this case, the clarifying agent composition is preferably applied from the surface of the substrate 10 opposite to the surface on which the barrier layer 20 is provided. This effectively prevents the aforementioned liquid clarifying agent composition from penetrating and leaking.

[0065] In a manufacturing method using a substrate 10 having a barrier layer 20, it is preferable to prepare a substrate 10 on which a barrier layer 20 is previously provided and use this. The method for forming the barrier layer 20 on the substrate is not particularly limited, and can be performed by a known method. For example, the barrier layer 20 can be formed using a coating machine such as a bar coater, a die coater, a gravure coater, a roll coater, a blade coater, an air knife coater, or a size press coater.

[0066] Subsequently, the substrate 10 is impregnated with the clarifying agent composition, and then the substrate 10 is dried. The solvent in the clarifying agent composition is evaporated and dried, causing the reactive components (a) and (b) to cure inside the substrate 10 and form a cured product. The drying method is not particularly limited, as long as it evaporates the solvent and creates a state in which the reaction between components (a) and (b) can be initiated. For example, depending on the types of components, natural drying may be used, or drying with a dryer at about 90 to 150°C may also be used. Heating may also be performed. The heating conditions may be any conditions that can promote the curing reaction. For example, the heating temperature is preferably 90 to 150°C, and more preferably 110 to 140°C. The heating time is preferably 0.5 to 4 minutes, and more preferably 1 to 2 minutes.

[0067] After heating, post-treatments such as humidification, pressing, calendering, annealing, corona treatment, etc. may be carried out as necessary, and known conditions may be adopted for these.

[0068] As described above, the transparent papers 1 and 2 according to the present embodiment have the advantages of excellent heat resistance and water resistance while maintaining sufficient transparency as transparent paper. Therefore, they can be suitably used as materials for various products that require visibility of contents or enclosed items, such as paper clear files, envelope paper, food packaging, and window materials for boxes and envelopes. Furthermore, because the transparent papers 1 and 2 according to the present embodiment are based on a paper substrate, they are also promising as a transparent alternative to plastic films, contributing to the recent demand for a plastic-free society. [Example]

[0069] The present invention will be described in more detail with reference to the following examples and comparative examples, but the present invention is not limited to these examples. Note that percentages and parts are by weight unless otherwise specified.

[0070] The materials used in this example are as follows: (base material) -Manufactured by Lintec Corporation, product name "(K) Shiro": Paper containing N and L pulp, not subjected to high-beating treatment or acid treatment, with a pulp beating degree of 20°SR, and a basis weight of 64 g / m 2 -Manufactured by Lintec Corporation, product name "Maru M Base Paper Shiro P": Paper containing N and L pulp, not subjected to high-beating treatment or acid treatment, with a beating degree of 25°SR, and a basis weight of 110 g / m 2 -Manufactured by Lintec Corporation, product name "Maru M Base Paper Shiro P": Paper containing N and L pulp, not subjected to high-beating treatment or acid treatment, with a beating degree of 25°SR, and a basis weight of 100 g / m 2 (clarifying agent component) Sorbitan monooleate: Kao Corporation, product name "Leodor SP-O10V"), freezing point -12.5℃ Sorbitan monooleate: Kao Corporation, product name "Leodor TW-O106V"), melting point 7.5℃ Glycerin fatty acid esters: Manufactured by Mitsubishi Chemical Corporation, product name "Ryoto™ Polyglycerol O-50D", a polyglycerol with two or more hydroxyl groups in the molecule, esterified with oleic acid (decaglycerol with an average degree of polymerization of approximately 10) Pentaerythritol monooleate: Kao Corporation, product name "Excepearl PE-MO" Sucrose fatty acid esters: Mitsubishi Chemical Corporation, product name "RYOTO™ Sugar Ester ER-190": sucrose fatty acid ester PVA: Kuraray Co., Ltd., product name "Kuraray Poval PVA 3-98", saponified polyvinyl alcohol Methylated melamine: Manufactured by Nippon Carbide Industries Co., Ltd., product name "Nicalac MW-30", weight average degree of polymerization 1.5 Polyfunctional isocyanates: Asahi Kasei Corporation, product name "Duranate™ MFA-75B", hexamethylene diisocyanate (difunctional isocyanate), butyl acetate as solvent, solid content 75% by mass p-Toluenesulfonic acid Showa Denko Materials, product name "Dryer 900"

[0071] Example 1

[0072] (Preparation of Clarifying Agent Composition) First, 100 parts by mass of sorbitan monooleate as component (a), 40 parts by mass of methylated melamine as component (b), and 10 parts by mass of a catalyst (p-toluenesulfonic acid) as component (d) were mixed with toluene to prepare a clarifying agent composition (coating liquid) with a solids concentration ("coating liquid concentration") of 66% by mass and a viscosity of 38 mPa s. The solids concentration of the coating solution ("coating solution concentration") refers to the proportion of non-volatile components in the clarifying agent composition, and non-volatile components are materials that do not volatilize under conditions of 50°C and 3 hours. The solids concentration was calculated from the mass of the non-volatile components and the mass of the solvent. The viscosity of the coating solution ("viscosity") was determined using a vibration viscometer.

[0073] (Making transparent paper) Next, a substrate shown in Table 1 was prepared as the pulp-containing substrate 10, and the coating liquid was applied to it using a Mayer bar with a cell volume selected to achieve the desired coating conditions. The substrate was then heated in a hot air drying oven at 140°C for 1 minute, curing components (a) and (b) inside the substrate 10 to form a clarifying agent. The dry adhesion weight of the clarifying agent was 17.2 g / m.2 and the proportion of the clarifying agent was 21 mass %. Transparent paper 1 (see FIG. 1) was obtained. Dry adhesion amount of the clarifying agent (g / m 2 ) refers to the dry adhesion amount of the clarifying agent per unit area of ​​the substrate 10. The ratio of the clarifying agent to the basis weight of the substrate 10 ("clarifying agent ratio", mass %) was calculated according to the following formula 1. Percentage of clarifying agent (mass%) = Dry adhesion amount of clarifying agent (g / m 2 ) / Basis weight of substrate (g / m 2 )×100...(Formula 1)

[0074] <Examples 2 to 7>

[0075] Transparent paper was produced in the same manner as in Example 1, except that the conditions were changed to those shown in Table 1.

[0076] <Comparative Examples 1 and 2>

[0077] The substrates shown in Table 1 were used as they were without being treated with a coating liquid.

[0078] <Comparative Examples 3 to 7>

[0079] Transparent paper was produced in the same manner as in Example 1, except that the conditions were changed to those shown in Table 1.

[0080] <Evaluation>

[0081] The properties of each of the obtained transparent papers were evaluated according to the methods described below.

[0082] (Opacity) Measurement was carried out in accordance with the measurement method of JIS P8149.

[0083] (Heat resistance) First, transparent paper was cut into a 30 mm square to prepare a sample. Then, the sample was stacked in the following order to prepare a test piece: glass plate (50 mm square, soda glass thickness 3 mm) / oil absorbing paper (50 mm square, commercially available (manufactured by Daiso Co., Ltd.)) / transparent paper (30 mm square) / PET film (50 mm square, PET thickness 50 μm) / weight (500 g). The test piece was then placed in a hot air oven at 160°C for 30 minutes. The test piece was then removed and allowed to cool to room temperature. The test piece was then peeled off, and visually inspected under fluorescent light to see if any components of the transparent paper had seeped onto the surface of the oil blotting paper that was in contact with the oil blotting paper sample (transparent paper). "Good": No leakage was observed. "X": Bleeding was observed.

[0084] (water resistance)

[0085] First, transparent paper was cut into 30 mm squares to prepare samples. The samples were then immersed in a tank of distilled water for 1 minute. The samples were then removed from the tank, and the moisture on the front and back surfaces of the paper was wiped off with a cloth. The water resistance was then evaluated based on the following evaluation criteria. "Good": The moisture was easily removed from the surface of the transparent paper using a cloth, and no deterioration of the paper surface was observed before or after immersion in water. "X": It was difficult to easily remove moisture from the surface of the transparent paper using a cloth, and the paper wiped after immersion in water had at least its surface damp with moisture.

[0086] Table 1 shows the production conditions for each example and comparative example, and Table 2 shows the evaluation results.

[0087] [Table 1]

[0088] [Table 2]

[0089] From the above, it was confirmed that this example at least provides transparent paper that has excellent heat resistance and water resistance while maintaining sufficient transparency as transparent paper. Furthermore, it was confirmed that even a substrate that has not been subjected to a high-beating degree treatment or an acid treatment can exhibit excellent physical properties, and restrictions on the selection of substrate materials can be alleviated. Furthermore, it was confirmed that the method for producing transparent paper is a simple method because it can omit the beating treatment, and can also contribute to reducing the beating power, i.e., the unit power consumption. [Explanation of symbols]

[0090] 1, 2: Transparent paper, 10: Base material, 20: Barrier layer

Claims

1. A method for producing transparent paper in which a pulp-containing substrate is impregnated with a clarifying agent, impregnating a substrate comprising pulp with a clarifying agent composition; drying the substrate; Including, The clarifying agent composition comprises: (a) an ester having two or more hydroxyl groups; (b) a melamine compound or an isocyanate compound having at least one group selected from the group consisting of a methylol group, a methoxy group, and an isocyanate group; and (c) an organic solvent; the mass ratio (b / a) of the component (b) to the component (a) is 15 mass% or more; a ratio of the clarifying agent to the basis weight of the substrate is 10% by mass or more; the step of impregnating the substrate with the clarifying agent composition is a step of applying the clarifying agent composition to the substrate, The substrate has a barrier layer provided on the surface opposite to the surface to which the clarifying agent composition is applied. How to make transparent paper.

2. The solid content concentration of the clarifying agent composition is 35% by mass or more and 70% by mass or less. The method for producing the transparent paper according to claim 1 .

3. The viscosity of the clarifying agent composition is 40 mPa s or less. The method for producing transparent paper according to claim 1 or 2.

4. The melamine compound is an alkylated melamine. The method for producing the transparent paper according to any one of claims 1 to 3.

5. The opacity of the transparent paper according to JIS P8149 is 60% or less. The method for producing the transparent paper according to any one of claims 1 to 4.

6. Transparent paper manufactured using the manufacturing method described in any one of claims 1 to 5.

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