JP7779677B2 - Release paper base paper, release paper, adhesive label, and method for producing release paper - Google Patents

Description

The present invention relates to release paper base paper, release paper, adhesive labels, and a method for producing release paper.

Adhesive labels are used to affix labels to transparent containers made of glass, plastic, etc. In these applications, transparency is required so that the adherend and the contents of the container underneath the label can be seen even after the label is affixed.

The release paper is a component whose function is to protect the adhesive layer of the adhesive label and prevent a decrease in adhesiveness. Because the release paper is positioned adjacent to the adhesive layer until it is attached to the adherend, the adhesive layer is prone to picking up any irregularities on the adhesive layer surface of the release paper. For this reason, irregularities on the release surface of the release paper can cause a decrease in the transparency of the adhesive label.

Therefore, in applications where transparency is required for adhesive labels, it is important that the release surface of the release paper is smooth. For example, Patent Document 1 discloses that the release surface of the release paper is made smooth by forming a pigment coating layer on a paper substrate and then forming a polyolefin layer on top of it. Release paper typically has a structure in which a release agent layer is formed on a release base paper.

JP 2013-230611 A

However, with laminated release paper with a polyolefin layer, as in Patent Document 1, the polyolefin layer is insoluble in water, making it difficult to recycle the release paper.

The present invention therefore aims to provide a release paper base paper that, when made into an adhesive label, improves the transparency of the adhesive label without using a polyolefin laminate.

The present invention relates to a release paper base paper comprising a substrate, coating layer 1, and coating layer 2 arranged in this order, the pulp used in the substrate containing hardwood pulp at a mass ratio of 50 mass% or more, the pulp used in the substrate having a beating degree of 30°SR or more, coating layer 1 containing resin, and coating layer 2 containing clay and a binder resin.

The release paper base paper of the present invention and the release paper containing said release paper base paper improve the transparency of adhesive labels when made into such labels without using a polyolefin laminate.

1 is a cross-sectional schematic view showing one embodiment of a release paper base paper. FIG. FIG. 2 is a cross-sectional schematic view showing another embodiment of the release paper base paper. FIG. 2 is a cross-sectional view showing an embodiment of a release paper. 1 is a cross-sectional view schematically illustrating an embodiment of an adhesive label.

In this specification, the range "X to Y" means "X or more and Y or less." Unless otherwise specified, operations and measurements of physical properties are performed at room temperature (20 to 25°C) and a relative humidity of 45 to 55% RH. Furthermore, in this specification, (meth)acrylic acid refers to both acrylic acid and methacrylic acid.

<Release paper base paper>
A first embodiment of the present invention is a release paper base paper comprising a substrate, a coating layer 1, and a coating layer 2 arranged in this order, the pulp used in the substrate containing hardwood pulp at a mass ratio of 50 mass% or more, the pulp used in the substrate having a beating degree of 30°SR or more, the coating layer 1 containing a resin, and the coating layer 2 containing clay and a binder resin.

Figure 1 is a cross-sectional schematic diagram illustrating the structure of a release paper base paper. Release paper base paper 10 is composed of a substrate 11, coating layer 1 12, and coating layer 2 13 arranged in this order. As shown in Figure 2, coating layer 1 may be formed on both sides of release paper base paper 10.

The basis weight of the release paper base paper is preferably 35 to 120 g/m ² , and more preferably 50 to 100 g/m ² .

The surface roughness of the release paper base paper on the coating layer 2 side is preferably 0.70 μm or less, more preferably less than 0.50 μm, in arithmetic mean roughness (Ra) according to JIS B 0601:2013. The lower limit of the arithmetic mean roughness (Ra) of the coating layer 2 is preferably as small as possible, but is typically 0.10 μm or more. In the present application, despite the extremely high smoothness of the release paper, the layer structure of the present application ensures adhesion to the release agent layer.

(Base material)
The mass percentage of hardwood pulp in the pulp used in the base material is 50% by mass or more. A hardwood pulp mass percentage of 50% by mass or more improves the smoothness of the release paper base paper surface. The mass percentage of hardwood pulp content is preferably more than 50% by mass, more preferably 60% by mass or more, even more preferably 70% by mass or more, particularly preferably more than 80% by mass, and most preferably 100% by mass.

Furthermore, from the viewpoint of smoothness, the mass ratio of hardwood pulp to the total amount of hardwood pulp and softwood pulp in the base material is preferably greater than 50% by mass, more preferably 60% by mass or more, even more preferably 70% by mass or more, particularly preferably greater than 80% by mass, and most preferably 100% by mass.

The hardwood pulp used as the base material is not particularly limited, and virgin pulp and recycled paper pulp can be used. It is preferable to use wood pulp as the virgin pulp. Examples of known hardwood virgin pulp include chemical pulps such as bleached hardwood kraft pulp (LBKP), unbleached hardwood kraft pulp (LUKP), semi-bleached hardwood kraft pulp (LSBKP), and sulfite hardwood pulp, as well as mechanical pulps such as stone ground pulp (SGP), pressed stone ground pulp (TGP), chemi-ground pulp (CGP), groundwood pulp (GP), and thermomechanical pulp (TMP). Among these, in terms of surface smoothness, chemical pulps such as bleached hardwood kraft pulp (LBKP), unbleached hardwood kraft pulp (LUKP), semi-bleached hardwood kraft pulp (LSBKP), and hardwood sulfite pulp are preferred, with bleached hardwood kraft pulp (LBKP) being more preferred.

Conventional softwood pulps that can be used for the base material include chemical pulps such as bleached softwood kraft pulp (NBKP), unbleached softwood kraft pulp (NUKP), semi-bleached softwood kraft pulp (NSBKP), and softwood sulfite pulp, as well as mechanical pulps such as stone ground pulp (SGP), pressed stone ground pulp (TGP), chemi-ground pulp (CGP), groundwood pulp (GP), and thermomechanical pulp (TMP).

In one preferred embodiment, when the content of softwood bleached kraft pulp is XN [mass%] and the content of hardwood bleached kraft pulp is XL [mass%], it is preferable that the relationship 0≦XN/XL≦1 is satisfied, more preferably that the relationship 0≦XN/XL<1 is satisfied, even more preferably that the relationship 0≦XN/XL≦3/7 is satisfied, even more preferably that the relationship 0≦XN/XL<2/8 is satisfied, and it is particularly preferable that XN/XL=0/100.

The pulp content of the base material is not particularly limited, but is preferably 60% by mass or more and 100% by mass or less. The lower limit of this content is more preferably 65% by mass or more, and even more preferably 70% by mass or more. The upper limit of this content is more preferably 98% by mass or less, and even more preferably 97% by mass or less.

The pulp used for the substrate has a degree of beating of 30°SR or higher. If the degree of beating of the pulp is less than 30°SR, the haze will be significantly higher when the adhesive label is made into a label (see comparison between Example 1 and Comparative Example 1 below). The detailed mechanism by which a pulp with a degree of beating of 30°SR or higher reduces the haze when the adhesive label is made into a label is unclear, but it is thought that a pulp with a degree of beating of 30°SR or higher makes it difficult for the coating liquid to sink into the substrate during wet coating to form coating layer 1 on the substrate, thereby ensuring surface smoothness after coating. Furthermore, if the pulp used for the substrate has a degree of beating of 30°SR or higher, the transparency of the release paper will be high. High transparency has the advantage of making it easier to detect the label when it is applied by machine, for example.

The degree of beating is preferably 40°SR or higher, and more preferably 45°SR or higher. There is no particular upper limit to the degree of beating, but in order to increase the papermaking speed, it is preferably 90°SR or lower, and more preferably 70°SR or lower. The degree of beating of the pulp can be controlled by the gap thickness of the double-disc refiner during beating.

The thickness of the substrate is preferably 40 μm or more and 300 μm or less. By making the thickness 40 μm or more, the occurrence of wrinkles during the production of the release paper can be suppressed, and the suitability for die-cutting during label production can be improved. Furthermore, by making the thickness 300 μm or less, the rigidity does not become too high, and handling properties do not deteriorate. This thickness can be measured in accordance with JIS P 8118:2014. The basis weight of the substrate is preferably 35 to 120 g/ ^m2 , and more preferably 50 to 100 g/ ^m2 . By controlling the basis weight within this range, the suitability for papermaking is further improved.

The base material may contain other additives. Examples of other additives include oxidized starch; sizing agents such as rosin-based sizing agents and AKD (alkyl ketene dimer)-based sizing agents; wettability enhancers; resin materials such as urethane resin, acrylic resin, styrene acrylic resin, and polyvinyl alcohol; colorants such as dyes and pigments; fixing agents such as aluminum sulfate, cationized starch, and cationic polyelectrolytes; papermaking aids; and flocculants. One or more selected from these may be used in combination. The above resin materials may also function as sizing agents.

(Coating layer 1)
Coat layer 1 is essential. If coat layer 1 is not present and only coat layer 2 is present, smoothness decreases (see Comparative Example 3 described below). This is thought to be because, without coat layer 1, the resin of coat layer 2 penetrates into the paper substrate, preventing coat layer 2 from fulfilling its proper role.

The coating layer 1 contains a resin. Examples of resins include copolymers such as styrene-butadiene copolymer, ethylene-vinyl chloride copolymer, methyl methacrylate-butadiene copolymer, ethylene-vinyl acetate copolymer, and (meth)acrylic acid ester copolymer; and water-soluble resins such as casein, dextrin, starch, oxidized starch, carboxymethyl cellulose, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, and polyvinyl alcohol. These resins may be used alone or in combination of two or more. Among these, in terms of adhesion to the release agent, it is preferable for the resin to contain polyvinyl alcohol, and it is particularly preferable for it to consist solely of polyvinyl alcohol.

Polyvinyl alcohol includes not only ordinary polyvinyl alcohol obtained by hydrolysis of polyvinyl acetate, but also modified polyvinyl alcohol. Examples of modified polyvinyl alcohol include cationically modified polyvinyl alcohol, anionically modified polyvinyl alcohol, and nonionically modified polyvinyl alcohol. Of these, ordinary polyvinyl alcohol obtained by hydrolysis of polyvinyl acetate is preferred.

The degree of saponification of polyvinyl alcohol is preferably 80 to 100 mol%, and more preferably 85 to 100 mol%. The molecular weight of polyvinyl alcohol is typically 5,000 to 150,000, and preferably 10,000 to 100,000.

The coating layer 1 may contain other additives. Examples of other additives include fillers such as clay, silica, calcium carbonate, titanium oxide, and zinc oxide, dispersants, thickeners, defoamers, anti-foaming agents, viscosity adjusters, lubricants, water-resistant agents, water retention agents, and colorants. A preferred embodiment is one in which the coating layer 1 does not contain a filler.

The surface to which Coating Layer 1 has been applied may be subjected to a supercalendering process to smooth the surface. Performing a supercalendering process improves the sealing and rigidity effects.

(Coating layer 2)
In the present invention, in addition to the coating layer 1, a coating layer 2 is disposed.

Coating layer 2 contains clay and a binder resin. The clay can fill in depressions that could not be sufficiently smoothed even with coating layer 1, improving the smoothness of the surface.

Examples of binder resins used in the coating layer 2 include copolymers such as styrene-butadiene copolymer, styrene-acrylic copolymer, ethylene-vinyl chloride copolymer, methyl methacrylate-butadiene copolymer, ethylene-vinyl acetate copolymer, and (meth)acrylic acid ester copolymer; and water-soluble resins such as casein, dextrin, starch, oxidized starch, carboxymethyl cellulose, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, and polyvinyl alcohol. These binder resins may be used alone or in combination. Among these, styrene-butadiene copolymer is preferred for its film-forming properties. Furthermore, styrene-butadiene resins (styrene-butadiene copolymers) are preferably latex-type (styrene-butadiene rubber latex) because of their high solids concentration, low viscosity, and excellent productivity. The glass transition temperature of styrene-butadiene resins is preferably -50°C or higher and 50°C or lower, and more preferably -20°C or higher and 20°C or lower, in terms of ease of smoothing by pressure in the supercalendering process. The minimum film-forming temperature of latex-type (styrene-butadiene rubber latex) is preferably in the range of 10 to 150°C. The minimum film-forming temperature is the lowest temperature required to form a continuous film when the water in the styrene-butadiene rubber latex evaporates and dries, and is obtained by the temperature gradient plate method. Commercially available styrene-butadiene copolymer latex-type products may be used, such as Smartex (registered trademark) SN-309R (styrene-butadiene rubber latex, manufactured by Nippon A&L Co., Ltd., glass transition temperature 4°C), Smartex (registered trademark) SN-307R (styrene-butadiene rubber latex, manufactured by Nippon A&L Co., Ltd., glass transition temperature 10°C), and JSR0693 (styrene-butadiene rubber latex, manufactured by JSR Corporation, glass transition temperature 20°C).

Furthermore, it is preferable to use a combination of starch and polyvinyl alcohol in addition to a styrene-butadiene copolymer as the binder resin. This combination improves film-forming properties and adhesion to the release agent layer, and also improves processability.

From the viewpoint of film-forming properties and adhesion to the release agent layer, the amount of starch added is preferably 40 to 70% by mass, and even more preferably 50 to 60% by mass, relative to the styrene-butadiene copolymer. The starch used can be made from corn, potato, tapioca, rice, etc. Modified starches such as oxidized starch, hydroxyethyl etherified starch, and oxygen-modified starch can also be used.

From the viewpoints of film-forming properties and adhesion to the release agent layer, the amount of polyvinyl alcohol added is preferably 5 to 40% by mass, more preferably 10 to 25% by mass, and even more preferably 13 to 20% by mass, relative to the styrene-butadiene copolymer. The same polyvinyl alcohols as those described in the Coating Layer 1 section can be used.

Examples of clay that can be used include kaolin, talc, bentonite, smectite, vermiculite, mica, chlorite, kibushi clay, gairome clay, halloysite, and mica. Among these, kaolin is preferred. The shape of the clay is not particularly limited, but from the perspective of smoothness, a flat plate shape is preferred. The average particle size of the clay is not particularly limited, but is, for example, 0.1 μm or more and 10 μm or less. The average particle size is measured by volume and can be measured using a laser diffraction particle size analyzer.

When the clay is flat and plate-shaped, the ratio of the average particle size to the average thickness, i.e., the aspect ratio, is preferably 3.0 or greater, more preferably 5.0 or greater, and even more preferably 7.0 or greater. The upper limit of the aspect ratio of the clay may be 10.0 or less.

From the standpoint of smoothness and binding properties, the clay content in coating layer 2 is preferably 60 to 85% by mass, and more preferably 70 to 80% by mass.

In the coating layer 2, the weight ratio of the binder resin to the clay is preferably 25% by weight or more, and more preferably 30% by weight or more. When the weight ratio of the binder resin to the clay is above the lower limit, the resin can adequately fill the gaps formed by the clay, suppressing penetration of the release agent layer applied on top and further improving smoothness. Taking into account the effects of the clay, the upper limit of the weight ratio of the binder resin to the clay is preferably 50% by weight or less, and more preferably 45% by weight or less.

The coating layer 2 may contain other additives. Examples of other additives include pigments, pigment dispersants, defoamers, anti-foaming agents, viscosity modifiers, lubricants, water-resistant agents, water retention agents, and colorants.

It is also preferable to subject the surface to supercalendering to smooth the surface after coating layer 2 has been applied. Supercalendering improves the sealing and rigidity effects.

(Manufacturing method)
One embodiment of a method for producing release paper base paper includes applying a resin-containing composition for forming coating layer 1 to a substrate having a hardwood pulp content of 50 mass % or more and a pulp beating degree of 30°SR or more to form coating layer 1, and applying a clay and a binder resin-containing composition for forming coating layer 2 to the substrate to form coating layer 2.

1. Preparation of the base material First, a pulp raw material is beaten in water to produce a pulp slurry, to which an internal additive is added as needed.

Next, the slurry containing the pulp obtained as described above and, if necessary, additives is made into a paper base material. If necessary, a sizing agent may be applied during papermaking. Examples of sizing agents that can be used include oxidized starch, styrene-acrylic resin, acrylic resin, and polyacrylamide resin. When a sizing agent is applied, it is usually applied to both sides of the paper, but it may also be applied to only one side.

2. Application of the composition for forming the coating layer 1 to the substrate A resin, an additive if necessary, and a solvent are mixed to prepare a composition for forming the coating layer 1. The solvent is appropriately selected depending on the form of the resin. When the resin is polyvinyl alcohol, the solvent is, for example, water.

The coating layer 1 is then formed by applying a coating layer 1-forming composition to the paper substrate obtained as described above. The coating method is not particularly limited, and conventional methods can be used, such as applying a coating layer 1-forming composition containing a resin component to a paper substrate and drying it to form the coating layer 1. The coating method is not particularly limited, and can be performed using an appropriate coating device, such as a blade coater, air knife coater, rod blade coater, bar blade coater, gravure coater, bar coater, multi-stage roll coater, roll coater, reverse roll coater, curtain coater, spray coater, gate roll coater, size press coater, or shim sizer. From the viewpoint of effectiveness, the coating amount (solid content) of the coating layer 1-forming composition (the amount applied to one side when coating both sides of a paper substrate) is preferably 0.2 to 4.0 g/m ² , and more preferably 0.5 to 3.0 g/m ² .

After the coating layer 1-forming composition has been applied, it may be subjected to a drying process. Drying conditions are set appropriately, but for example, drying at 80 to 160°C for 10 to 60 seconds.

3. Application of the composition for forming the coating layer 2 to the substrate The composition for forming the coating layer 2 is prepared by mixing a binder resin, clay, and, if necessary, additives and a solvent. The solvent is appropriately selected depending on the form of the binder resin. When the binder resin is a latex-type resin or a water-soluble polymer, the solvent is preferably water or an alcohol (e.g., ethanol, isopropanol, etc.), and more preferably water. The solvent may be used alone or in combination of two or more.

Then, a composition for forming coating layer 2 is applied to the surface of the paper substrate on which the coating layer 1 obtained as described above has been formed, to form coating layer 2. Examples of application methods include the same methods as those for applying the composition for forming coating layer 1 described above.

From the viewpoint of effectiveness, the amount of coating (solid content) of the composition for forming the coating layer 2 is preferably 2 to 30 g/m ² , more preferably 3 to 20 g/m ^2. After the composition for forming the coating layer 2 is applied, it may be subjected to a drying step. The drying conditions are appropriately set, but are, for example, 80 to 160°C for 10 to 60 seconds.

<Release paper>
3 is a cross-sectional view showing the structure of a release paper 30. The release paper 30 of FIG. 3 is formed by forming a release agent layer 14 on the coating layer 2 of the release paper base paper 10 of FIG.

Examples of release agents that form the release agent layer include silicone-based release agents, rubber-based elastomers such as olefin-based resins, isoprene-based resins, and butadiene-based resins, long-chain alkyl-based resins, long-chain alkyl acrylate-based resins, alkyd-based resins, and fluorine-based resins. Of these, silicone-based release agents are preferred. The thickness of the release agent layer is typically around 0.01 to 5 μm.

Examples of silicone-based release agent resins include homopolymers or copolymers of trimethylchlorosilane, dimethyldichlorosilane, methyltrichlorosilane, diphenyldichlorosilane, phenyltrichlorosilane, and methylvinyldichlorosilane. These can be used alone or in combination of two or more. Silicone-based release agents can be used in any of the following forms: solvent-based, solventless, and emulsion-based. From an environmental perspective, emulsion-based or solventless-based types are preferred.

The release agent layer 13 preferably has a thickness of 0.01 μm or more and 10 μm or less, and more preferably 0.1 μm or more and 5 μm or less.

There are no particular limitations on the method for applying the release agent to the coating layer 2, and various coating devices can be appropriately selected and used, such as a blade coater, air knife coater, rod blade coater, bar blade coater, gravure coater, bar coater, or multi-stage roll coater.

From the viewpoint of smoothness after application, the amount of release agent applied (solid content) is preferably 0.1 g/ ^m2 or more, more preferably more than 0.4 g/ ^m2 , and even more preferably more than 1.0 g/ ^m2 . Furthermore, from the viewpoint of suppressing the occurrence of blocking during winding, the amount of release agent applied is preferably 2.5 g/ ^m2 or less, and more preferably 2.0 g/ ^m2 or less.

The surface roughness of the release paper on the release agent layer side is preferably 0.70 μm or less, more preferably 0.65 μm or less, even more preferably 0.5 μm or less, and particularly preferably less than 0.5 μm, in terms of arithmetic mean roughness (Ra) in accordance with JIS B 0601:2013. The lower limit of the arithmetic mean roughness (Ra) of the release agent layer is preferably as small as possible, but is typically 0.10 μm or more.

<Adhesive label>
4 is a cross-sectional view showing a schematic configuration of an adhesive label. An adhesive label 40 in FIG. 4 further includes an adhesive layer 15 and a label substrate 16 in this order from the release agent layer side of the release paper 30 in FIG.

(Label substrate)
The label substrate is not particularly limited and may be a resin substrate or a paper substrate. The label substrate is preferably a transparent substrate, since the release paper of the above embodiment has high smoothness and can reduce the haze of the adhesive label. Here, "transparent" means that the transmittance in the visible light range is 80% or more, preferably 90% or more (upper limit 100%). The transmittance of the transparent substrate can be measured according to JIS K7361-1:1997 (Test method for total light transmittance of plastic-transparent materials).

The material constituting the transparent substrate is not particularly limited, and examples include polyester films such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; polyolefin films such as polyethylene and polypropylene; cellophane, diacetyl cellulose film, triacetyl cellulose film, acetyl cellulose butyrate film, polyvinyl chloride film, polyvinylidene chloride film, polyvinyl alcohol film, polystyrene film, polycarbonate film, polymethylpentene film, polysulfone film, fluororesin film, acrylic resin film, norbornene resin film, and cyclic olefin film. Among these, polyester and polyolefin are preferred in terms of transparency, with polyethylene terephthalate, polyethylene, and polypropylene being more preferred, and polyethylene terephthalate being even more preferred.

The label substrate may contain stabilizers, lubricants, softeners, UV absorbers, antioxidants, antistatic agents, and flame retardants as needed. The resin substrate may also have an easy-adhesion layer to improve adhesion of the printed layer.

There are no particular restrictions on the thickness of the label substrate, but it is preferable that it be between 5 and 250 μm.

(Adhesive layer)
The adhesive used in the adhesive layer is not particularly limited, and may be an acrylic adhesive, a rubber adhesive, a silicone adhesive, a urethane adhesive, a polyester adhesive, a vinyl alkyl ether adhesive, etc. The above adhesives may be used alone or in combination of two or more.

From the perspective of adhesive reliability, acrylic adhesives are particularly suitable as adhesives. The acrylic polymer that makes up the acrylic adhesive is formed by using an adhesive (meth)acrylic acid alkyl ester as the main monomer component, and, as necessary, a monomer (copolymerizable monomer) that can be copolymerized with the (meth)acrylic acid alkyl ester that has cohesive properties, and a copolymerizable monomer that has a crosslinkable functional group.

Examples of adhesive (meth)acrylic acid alkyl esters include butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and isooctyl (meth)acrylate. These may be used alone or in combination of two or more.

Examples of copolymerizable monomers having aggregating properties include methyl (meth)acrylate, vinyl acetate, styrene, and acrylonitrile. These may be used alone or in combination of two or more.

Examples of copolymerizable monomers having a crosslinkable functional group include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and (meth)acrylic acid. These may be used alone or in combination of two or more.

The weight-average molecular weight of the acrylic polymer is not particularly limited, but is preferably between 100,000 and 1,000,000. The weight-average molecular weight is a polystyrene-equivalent value measured by gel permeation chromatography (GPC).

The adhesive preferably contains a crosslinking agent in addition to the acrylic polymer. Examples of crosslinking agents include isocyanate-based crosslinking agents, epoxy-based crosslinking agents, and metal chelate-based crosslinking agents. The amount of crosslinking agent added is preferably 0.001 to 10 parts by mass, and more preferably 0.005 to 2.0 parts by mass, per 100 parts by mass of the acrylic polymer.

If necessary, fillers, light stabilizers, antioxidants, colorants, antistatic agents, tackifiers, wetting agents, leveling agents, thickeners, antifoaming agents, preservatives, etc. may be added to the adhesive layer.

The thickness of the adhesive layer is not particularly limited, but from the standpoint of adhesiveness and thin film formation, a thickness in the range of 10 to 100 μm is preferred.

The method for forming the adhesive layer is not particularly limited, but typically involves applying the adhesive to the release layer of the release paper, drying it, and then transferring it to the label substrate. The application method is not particularly limited, and it can be done using known application equipment such as a roll coater, knife coater, air knife coater, bar coater, blade coater, slot die coater, lip coater, or gravure coater. Drying conditions are not particularly limited, and are typically done at 60-150°C for 10-60 seconds.

(Printing layer)
The pressure-sensitive adhesive label may have, for example, a printed layer on a label substrate. There are no particular limitations on how the printed layer may be formed, and it may be formed by, for example, flexographic printing, offset printing, letterpress printing, gravure printing, screen printing, or the like.

The information displayed on the printing layer may be, for example, letters, numbers, illustrations, photographs, graphs, etc., or a combination thereof.

The effects of the present invention will be explained using the following examples and comparative examples. In the examples, the terms "parts" and "%" may be used, but unless otherwise specified, they represent "parts by mass" or "% by mass." Furthermore, unless otherwise specified, all operations are performed at room temperature (25°C).

Example 1
1. Preparation of Paper-Based Substrate Hardwood bleached kraft pulp (LBKP) was beaten in water to a beating degree of 42°SR according to the Schopper-Rigler method, and dispersed in water to obtain a pulp dispersion with a concentration of approximately 1.0% by mass. To 100 parts by mass of the pulp in this pulp dispersion, 0.4 parts by mass of rosin sizing agent and 0.2 parts by mass of aluminum sulfate fixing agent were added to obtain a pulp slurry. This pulp slurry was then made into a paper substrate with a basis weight of 59 g/ ^m2 using a Fourdrinier multi-cylinder paper machine. The thickness of the paper substrate was 49 μm.

2. Preparation of release paper base paper A composition containing polyvinyl alcohol (saponification degree 98 mol%, molecular weight 75,000) and water in a specified ratio was applied to one side of the paper substrate obtained in 1. so that the coating amount was 1.0 g/ ^m2 , and the coating was dried to form a coating layer 1. The thickness of the formed coating layer 1 was 1 μm.

Next, a coating liquid was prepared containing flat plate-like kaolin (average particle size 4.3 μm), oxidized starch, polyvinyl alcohol (saponification degree 98 mol%, molecular weight 75,000), and styrene-butadiene resin (styrene-butadiene rubber latex, glass transition temperature -3°C, minimum film formation temperature 100°C) in the proportions (solid content) shown in Table 1 below, and water as a solvent in a specified proportion. Next, the coating liquid was applied to coating layer 1 so as to achieve a coating amount of 5.2 g/ ^m2 , dried, and then passed through a supercalender roll to form coating layer 2, thereby obtaining a release paper base paper. The thickness of the formed coating layer 2 was 5 μm.

3. Preparation of release paper A silicone-based release agent (KNS-3051 manufactured by Shin-Etsu Chemical Co., Ltd.) was applied to the coating layer 2 of the release paper base paper obtained in 2. in an amount of 1.7 g/ ^m2 , and cured at 150°C for 30 seconds to form a release agent layer, thereby obtaining release paper.

4. Preparation of Adhesive Label An acrylic adhesive composition was obtained by mixing 100 parts by mass (solid content) of an acrylic resin (n-butyl acrylate/acrylic acid=90/10), 30 parts by mass (solid content) of a rosin ester tackifier (softening point: 100°C), and 1.5 parts by mass (solid content) of an isocyanate crosslinking agent (Coronate L manufactured by Tosoh Corporation).

The above pressure-sensitive adhesive composition was applied to the release paper obtained in 3. so that the application amount was 20 g/ ^m2 . Next, the pressure-sensitive adhesive layer surface was attached to a PET substrate (thickness: 50 μm) which is a transparent resin substrate, and the resulting mixture was left to stand at 23° C. for 1 week to prepare a pressure-sensitive adhesive label.

Example 2
A release paper base paper, a release paper, and an adhesive label were obtained in the same manner as in Example 1, except that hardwood bleached kraft pulp (LBKP) was beaten in water to a beating degree of 49°SR according to the Schopper-Rigler method.

Example 3
A release paper base paper, release paper, and adhesive label were obtained in the same manner as in Example 1, except that the silicone-based release agent (KNS-3051 manufactured by Shin ^- Etsu Chemical Co., Ltd.) was applied in an amount of 1.0 g/m².

Example 4
A release paper base paper, a release paper, and an adhesive label were obtained in the same manner as in Example 1, except that the silicone-based release agent (KNS-3051 manufactured by Shin ^- Etsu Chemical Co., Ltd.) was applied in an amount of 0.4 g/m2.

Example 5
A release paper base paper, a release paper, and an adhesive label were obtained in the same manner as in Example 1, except that the amount of bleached hardwood kraft pulp (LBKP) was changed from 100 parts by mass to 30 parts by mass of bleached softwood kraft pulp (N-BKP) and 70 parts by mass of bleached hardwood kraft pulp (LBKP), and the amount of coating layer 2 was changed to 4.4 g/ ^m2 .

Example 6
A release paper base paper, a release paper, and an adhesive label were obtained in the same manner as in Example 1, except that 100 parts by mass of hardwood bleached kraft pulp (LBKP) was replaced with 50 parts by mass of softwood bleached kraft pulp (N-BKP) and 50 parts by mass of hardwood bleached kraft pulp (LBKP).

(Comparative Example 1)
A release paper base paper, a release paper, and an adhesive label were obtained in the same manner as in Example 1, except that hardwood bleached kraft pulp (LBKP) was beaten in water to a beating degree of 23°SR according to the Schopper-Rigler method.

(Comparative Example 2)
A release paper base paper, a release paper and an adhesive label were obtained in the same manner as in Example 1, except that the coating layer 2 was not provided.

(Comparative Example 3)
A release paper base paper, a release paper and an adhesive label were obtained in the same manner as in Example 1, except that the coating layer 1 was not provided.

(Evaluation Method 1: Arithmetic Mean Roughness Ra)
The arithmetic mean roughness Ra of the release agent layer surface of the release paper was measured using a contact surface roughness meter (SV-3000, manufactured by Mitutoyo Corporation) in accordance with JIS B0601: 2013. The results are shown in Table 1 below.

(Evaluation method 2: Haze)
The haze was measured after peeling the release paper from each adhesive label. The haze was measured using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., NDH 5000) in accordance with JIS K7136:2000. The results are shown in Table 1 below.

The release paper base paper and release paper produced in the examples were highly smooth. Furthermore, the adhesive labels of Examples 1 to 6 were shown to have high transparency. On the other hand, the adhesive labels of Comparative Example 1, which had a beating degree of less than 30°SR, Comparative Example 2, which did not have coating layer 2 formed, and Comparative Example 3, which did not have coating layer 1 formed, had low transparency.

10 Release paper base paper,
11 base material,
12 Coating layer 1,
13 Coat layer 2,
14 release agent layer,
15 adhesive layer,
16 label substrate,
30 Release paper.

Claims (8)

a substrate, a coating layer 1, and a coating layer 2 are arranged in this order;
The content by mass of hardwood pulp in the pulp used in the base material is 50% by mass or more,
The beating degree of the pulp used in the base material is 30°SR or more,
The coating layer 1 contains a resin,
The release paper base paper, wherein the coating layer 2 contains clay and a binder resin,
the binder resin contained in the coating layer 2 is a combination of a styrene-butadiene copolymer, starch, and polyvinyl alcohol;
The release paper base paper , wherein the content of the clay in the coating layer 2 is 60 to 85% by mass . The release paper base paper according to claim 1 , wherein the resin contained in the coating layer (1) includes polyvinyl alcohol. 3. The release paper base paper according to claim 1 , wherein the beating degree is 40°SR or more. The release paper base paper according to any one of claims 1 to 3 , wherein the ratio of the binder resin to the clay contained in the coating layer 2 is 25% by mass or more. A release paper base paper according to any one of claims 1 to 4 ;
a release agent layer laminated on at least a portion of the surface of the coating layer 2 of the release paper base paper;
including a release paper. An adhesive label having the release paper according to claim 5 . A coating layer 1 is formed by applying a resin-containing composition for forming a coating layer 1 onto a substrate having a hardwood pulp content of 50% by mass or more and a pulp beating degree of 30°SR or more;
and applying a composition for forming a coating layer (2) containing clay and a binder resin onto the coating layer (1) to form a coating layer (2).
A method for producing release paper base paper,
the binder resin contained in the composition for forming the coating layer 2 is a combination of a styrene-butadiene copolymer, starch, and polyvinyl alcohol;
The method for producing a release paper base paper , wherein the content of the clay in the coating layer 2 is 60 to 85% by mass . The method for producing a release paper base paper according to claim 7 , wherein the coating amount (solid content) of the composition for forming the coating layer 2 is 2.0 g/m ² or more.