JP2024013940A - Backup board and its manufacturing method - Google Patents

Abstract

[Problem] To provide a backup board in which paper does not easily peel off no matter which direction the adhesive tape is peeled off. [Solution] Two or more paper base materials, an adhesive layer made of a water-soluble adhesive that adheres the paper base materials, and a pigment coating layer on at least one surface, the pigment coating layer having an aspect ratio. Contains 0.1 parts by weight or more and 30 parts by weight or less of a water-soluble or water-dispersible binder per 100 parts by weight of a flat pigment with a ratio of 5 or more, and has a hardness of 75 as measured with a type D durometer according to JIS K6253. That's all for the backup board. [Selection diagram] None

Description

The present invention relates to a backup board that is placed under a workpiece during machining such as drilling, milling, and grinding, and a method for manufacturing the same.

During machining such as drilling, milling, and grinding, a backup board is sometimes placed under the workpiece to prevent damage to the stage caused by tools penetrating the workpiece and to improve machining accuracy. . For example, drilling, which is one of the manufacturing processes for printed wiring boards, is generally performed with an entry board, multiple boards that will become printed wiring boards, and a backup board stacked in this order from the drill entry side. The backup board placed under the substrate prevents the stage from being damaged by a drill blade that penetrates the substrate, and also reduces the occurrence of burrs at the hole-drilling portion of the substrate. At this time, the workpieces such as the entry board, the substrate, and the backup board are pasted together with adhesive tape and machined in an integrated state to prevent positional displacement during processing. Further, the backup board is used by bonding one side to a workpiece, then turning it over and bonding the other side to the workpiece.

The present inventors have proposed a recyclable and inexpensive backup board that is mainly made of paper (Patent Document 1). In the backup board described in Patent Document 1, when the bonded adhesive tape is peeled off from the main surface side of the backup board, the paper is less likely to peel off. However, when the adhesive tape is removed from the end (side) side of the backup board, the paper may peel off.
If the paper peels off at a deep position, a large step will occur on the surface of the backup board. If the adhesive tape is peeled off after using one side and the backup board with a large step is turned over and used, the other side with the step will be attached to the workpiece. In this case, a space is created between the workpiece and the backup board due to the difference in level of the backup board, so that burrs and the like may occur during machining.

Patent No. 6961322

An object of the present invention is to provide a backup board in which the paper does not easily peel off no matter which direction the adhesive tape is peeled off.

Means for solving the problems of the present invention are as follows.
1. Two or more paper base materials, an adhesive layer made of a water-soluble adhesive that adheres the paper base materials, and a pigment coating layer on at least one surface,
The pigment coating layer contains 0.1 parts by weight or more and 30 parts by weight or less of a water-soluble or water-dispersible binder based on 100 parts by weight of a flat pigment having an aspect ratio of 5 or more,
A backup board characterized by having a hardness of 75 or more as measured with a type D durometer in accordance with JIS K6253.
2. 1. A coating amount of the pigment coating layer is 4 g/m ² or more. Backup board as described in .
3. 1. The flat pigment has a Mohs hardness of 5 or less. or 2. Backup board as described in .
4. 1. The water-soluble or water-dispersible binder is polyvinyl alcohol or starch. or 2. Backup board as described in .

The backup board of the present invention has a pigment coating layer on at least one surface thereof. This pigment coating layer is the weakest point in the thickness direction of the backup board, and when the adhesive tape is peeled off, the pigment coating layer will break. In the backup board of the present invention, peeling occurs not in the paper but in the pigment coating layer, so the depth of the step that occurs when the adhesive tape is peeled off is very shallow and is within the thickness of the pigment coating layer. Even when the resulting surface is bonded to a workpiece and machined, defects such as burrs are less likely to occur.
The backup board of the present invention has sufficient hardness and can be used as a substitute for conventionally used baked boards, fiber boards, aluminum metal plates, and the like. The backup board of the present invention is less expensive than conventional baking boards and the like. Since the backup board of the present invention can be disintegrated, it can be recycled as waste paper, reducing disposal costs and reducing the burden on the environment.
The backup board of the present invention, in which the flat pigment contained in the pigment coating layer has a Mohs hardness of 5 or less, can suppress tool wear.

The details of the present invention will be explained below.
The backup board of the present invention has two or more paper base materials, an adhesive layer made of a water-soluble adhesive that adheres the paper base materials, and a pigment coating layer on at least one surface, and the pigment coating layer is , containing 0.1 parts by weight or more and 30 parts by weight or less of a water-soluble or water-dispersible binder per 100 parts by weight of a flat pigment with an aspect ratio of 5 or more, and hardness measured with a type D durometer in accordance with JIS K6253. is 75 or more.
The backup board of the present invention is used for machining such as drilling, milling, and grinding, and is particularly suitable for drilling.

<Paper base material>
In the present invention, the paper base material is a sheet made of pulp, filler, various auxiliary agents, and the like.
Pulps include bleached softwood kraft pulp (NBKP), unbleached softwood kraft pulp (NUKP), bleached hardwood kraft pulp (LBKP), unbleached hardwood kraft pulp (LUKP), sulfite pulp (SP), etc. Chemical wood pulp, ground pulp (GP), refined ground pulp (RGP), stone ground pulp (SGP), chemical ground pulp (CGP), semi-chemical pulp (SCP), thermomechanical pulp (TMP), chemi-thermomechanical Waste paper that is made from wood mechanical pulp such as wood pulp (CTMP), non-wood pulp obtained from kenaf, bagasse, bamboo, hemp, straw, etc., and waste paper, and the ink contained in these waste papers is removed through a deinking process. It is possible to appropriately mix and use known pulps such as pulp.

If foreign matter gets mixed into the paper base material, it will cause damage and wear to tools such as drills, so it is preferable to use chemical pulp such as LBKP or NBKP, which is less likely to get mixed in with foreign matter. Furthermore, since there is a risk of foreign matter being mixed in, it is important to note that among the two or more paper base materials constituting the backup board of the present invention, at least the two paper base materials located on the outermost surface should contain waste paper pulp relative to the total pulp. The content is preferably 1.0% by weight or less, more preferably 0.5% by weight or less, and most preferably not included (blended).

The freeness (Canadian standard freeness (hereinafter also referred to as CSF)) of the pulp used is preferably 600 ml or less, more preferably 540 ml or less, and even more preferably 480 ml or less.
In addition, as will be explained in detail in <Method for manufacturing backup board> below, especially when press treatment is not performed during backup board manufacture, the freeness of the pulp used is preferably 480 ml or less, and 440 ml or less. More preferably, the volume is 400 ml or less.
In addition, when the paper base material uses multiple types of pulps, the multiple types of pulps may be beaten separately or may be mixed and beaten. When beating separately, the freeness of each pulp is not particularly limited, but it is adjusted so that the freeness of the pulp slurry after mixing the pulps satisfies the above conditions.

Fillers include talc, kaolin, calcined kaolin, clay, heavy calcium carbonate, light calcium carbonate, white carbon, zeolite, magnesium carbonate, barium carbonate, titanium dioxide, zinc oxide, silicon oxide, amorphous silica, aluminum hydroxide. , inorganic fillers such as calcium hydroxide, magnesium hydroxide, zinc hydroxide, barium sulfate, and calcium sulfate, and organic fillers such as urea-formalin resin, polystyrene resin, phenol resin, and micro hollow particles. I can do it.
In addition, in the backup board of the present invention, the paper base material does not need to contain filler, and in order to prevent wear of tools such as drills, among the two or more paper base materials constituting the backup board of the present invention, The filler content in at least the two paper base materials located on the outermost surface is preferably 1.0% by weight or less, more preferably 0.5% by weight or less, and is preferably not contained (blended). Most preferred.

Various auxiliary agents include various internal sizing agents such as rosin, alkyl ketene dimer (AKD), and alkenylsuccinic anhydride (ASA), various nonionic, cationic, and amphoteric retention improvers, and freeness improvers. , paper strength improvers, various starches, polyacrylamide, urea resin, melamine resin, epoxy resin, polyamide resin, polyamine resin, polyethylene imine, vegetable gum, polyvinyl alcohol, latex, polyethylene oxide, hydrophilic crosslinked polymer particle dispersion, and Derivatives or modified products of these, basic aluminum compounds such as aluminum sulfate, aluminum chloride, sodium aluminate, basic aluminum chloride, basic polyaluminum hydroxide, water-soluble aluminum compounds such as alumina sol that easily decomposes in water, Examples include polyvalent metal compounds such as ferrous sulfate and ferric sulfate, silica sol, antifoaming agents, colored dyes, optical brighteners, pH adjusters, pitch control agents, slime control agents, etc. It can be selected and used as appropriate.

The backup board of the present invention has two or more paper base materials, and the raw material, basis weight, density, thickness, etc. of each paper base material may be the same or different.
In the backup board of the present invention, at least one of the paper substrates used preferably has a pulp disintegration freeness (Canadian standard freeness) of 480 ml or less, more preferably 440 ml or less, More preferably, the volume is 400 ml or less. A paper base material with a low pulp freeness is hard due to the formation of strong hydrogen bonds, which results in an improvement in the hardness of the backup board. Note that the disintegrated freeness is the Canadian standard freeness measured in accordance with JIS P8121 by disintegrating the paper base material by the method specified in JIS P8220-1. In the backup board of the present invention, it is more preferable that half or more of the paper base materials used satisfy the above-mentioned disintegration freeness, and it is even more preferable that all of the paper base materials satisfy the above-mentioned disintegration freeness.

If the disaggregated freeness of the pulp of all paper base materials is not 480 ml or less, the mixed disaggregated freeness represented by the following formula (1) is preferably 480 ml or less, more preferably 440 ml or less. , more preferably 400 ml or less.
Formula (1)
Mixed disintegration freeness = f1 x (g1/G) + f2 x (g2/G) +...
+fn×(gn/G)
(However, n represents the number of paper base materials,
f1, f2, ... fn are the disintegration freeness of paper base materials 1 to n, respectively;
g1, g2,...gn are the basis weights of paper base materials 1 to n, respectively;
G represents the sum of the basis weights of paper base materials 1 to n (G=g1+g2+...+gn). )

If the pulp freeness of all paper base materials is not 480 ml or less, the pulp freeness of the paper base whose pulp freeness exceeds 480 ml is preferably 600 ml or less, and preferably 560 ml or less. More preferably, the amount is 520 ml or less.

If the pulp freeness of all the paper base materials is not 480 ml or less, the number of paper base materials with a pulp freeness exceeding 480 ml is preferably 2 sheets or less, and more preferably 1 sheet. preferable. In particular, when three or more paper base materials are bonded together, it is preferable that half or more of the paper base materials have a pulp disintegration freeness of 480 ml or less, more preferably 65% or more, and 80% or more. is even more preferable.

The basis weight of at least one paper base material is preferably 150 g/m ² or more, more preferably 200 g/m ² or more, and even more preferably 250 g/m ² or more. If the basis weight of all the paper base materials is less than 150 g/m ² , the number of paper base materials required to form the backup board of the present invention will increase. The upper limit of the basis weight of the paper base material is not particularly limited as long as it can be wound into a roll, but it is preferably 650 g/m ² or less, more preferably 600 g/m ² or less, More preferably, it is 550 g/m ² or less. Further, in the present invention, it is more preferable that half or more of the paper base materials used satisfy the above basis weight, and it is even more preferable that all of the paper base materials satisfy the above basis weight.
Furthermore, when three or more paper base materials are laminated together, the ratio of the total basis weight of paper base materials with a disintegration freeness of 480 ml or less to the total basis weight of the paper base materials used is 50% by weight. % or more, more preferably 65% by weight or more, even more preferably 80% by weight or more.

The density of at least one paper base material is preferably 0.7 g/cm ³ or more, more preferably 0.75 g/cm ³ or more, and preferably 0.8 g/cm ³ or more. More preferred. Further, its density is preferably 1.2 g/cm ³ or less, more preferably 1.15 g/cm ³ or less, and even more preferably 1.1 g/cm ³ or less.
When the density of at least one paper base material is within the above range, a backup board that satisfies a predetermined hardness is likely to be obtained. Further, in the present invention, it is more preferable that half or more of the two or more paper base materials used satisfy the above density, and it is even more preferable that all of the paper base materials satisfy the above density.

The paper making method of the paper base material and the type of paper machine are not particularly limited, and include fourdrinier paper machines, twin wire paper machines, cylinder paper machines, gap formers, hybrid formers (on-top formers), etc. Known manufacturing (paper making) methods and paper machines such as a multi-layer paper machine and a combination of these can be selected. Among these, it is preferable to use a single-layer paper machine. A paper base material produced by multilayer paper making using a multilayer paper machine may have a weak interlaminar strength. In addition, backup boards are sometimes used by attaching them to machine tool stages, workpieces, entry boards, etc. with adhesive tape, but the multilayer paper base material is used when the adhesive tape is removed. Peeling may occur from the joint.
In addition, the pH during papermaking may be any of the acidic region (acidic papermaking), pseudo-neutral region (pseudo-neutral papermaking), neutral region (neutral papermaking), and alkaline region (alkaline papermaking). Afterwards, an alkaline chemical may be applied to the surface of the paper base material. Furthermore, water squeezing, drying, size press, calender, etc. after the wire part can be performed by methods known in the paper manufacturing field.

The paper base material is preferably calendered or supercalendered. By calendering or supercalendering, the paper base material is strongly crushed and its hardness is improved, so it is easy to make the backup board even higher in hardness.

<Water-soluble adhesive>
The water-soluble adhesive for bonding the paper base material is not particularly limited, and includes fully saponified polyvinyl alcohol, partially saponified polyvinyl alcohol, carboxy-modified polyvinyl alcohol, diacetone-modified polyvinyl alcohol, acetoacetyl-modified polyvinyl alcohol, amide-modified polyvinyl alcohol, Polyvinyl alcohols such as sulfonic acid-modified polyvinyl alcohol, butyral-modified polyvinyl alcohol, olefin-modified polyvinyl alcohol, nitrile-modified polyvinyl alcohol, pyrrolidone-modified polyvinyl alcohol, silanol-modified polyvinyl alcohol, cation-modified polyvinyl alcohol, terminal alkyl-modified polyvinyl alcohol, starch, enzymes Starches such as modified starch, thermochemically modified starch, oxidized starch, esterified starch, etherified starch (for example, hydroxyethylated starch, etc.), cationized starch, hydroxyethyl cellulose, methyl cellulose, ethyl cellulose, carboxymethyl cellulose, acetyl cellulose, etc. Cellulose ether and its derivatives can be used. Furthermore, a heat-melting water-soluble adhesive that can be subjected to heat treatment such as heat sealing can also be used. Since the backup board of the present invention uses a water-soluble adhesive as the adhesive for bonding the paper base material, it is easy to disintegrate and can be recycled as used paper.

<Pigment coating layer>
The backup board of the present invention includes, on at least one side, 0.1 parts by weight or more and 30 parts by weight of a water-soluble or water-dispersible binder (hereinafter also referred to as binder) per 100 parts by weight of a flat pigment having an aspect ratio of 5 or more. It has a pigment coating layer containing less than 50% of the pigment coating layer. Pigment coating layers can also be provided on both sides of the backup board.
The pigment coating layer contains a flat pigment and has a small content of binder with respect to the flat pigment. Therefore, in the pigment coating layer, the flat pigments are spread so that their flat surfaces are substantially parallel to the main surface of the backup board, but since the amount of binder is small, the bond between the flat pigments is weak. As a result, in the pigment coating layer of the present invention, the bond between the flat pigments laminated in the thickness direction is weak, and the strength is the lowest in the thickness direction of the backup board. becomes.

- Flat pigment The aspect ratio (major axis/thickness) of the flat pigment is 5 or more. When the aspect ratio of the flat pigment is 5 or more, the flat pigment tends to exist in layers in the thickness direction with the flat surfaces arranged substantially parallel to the main surface. The aspect ratio of the flat pigment is preferably 8 or more, more preferably 10 or more. The upper limit of this aspect ratio is not particularly limited, but is about 200 or less.
The volume 50% average particle diameter (D50) (hereinafter also referred to as "average particle diameter") of the flat pigment is not particularly limited, but is preferably 5 μm or more.
It is preferable that the flat pigment has a Mohs hardness of 5 or less in order to prevent tool wear. The Mohs hardness of the flat pigment is more preferably 4 or less, and even more preferably 3 or less.

Flat pigments having an aspect ratio (major axis/thickness) of 5 or more can be used without particular limitation, such as kaolin, clay, engineered kaolin, delaminated clay, heavy calcium carbonate, and light pigments. Inorganic pigments such as calcium carbonate, mica, talc, titanium dioxide, barium sulfate, calcium sulfate, zinc oxide, silicic acid, silicates, colloidal silica, satin white, and organic pigments such as dense, hollow, or core-shell types. Among them, one type of flat type or a mixture of two or more types can be used. When two or more types of flat pigments are used, the aspect ratio, average particle diameter, and Mohs hardness of each flat pigment may be the same or different.

- Water-soluble or water-dispersible binder A water-soluble or water-dispersible binder binds flat pigments together.
As the water-soluble binder, one kind or a mixture of two or more of the polyvinyl alcohols, starches, cellulose ethers, derivatives thereof, etc. listed as the above-mentioned water-soluble adhesives can be used.
As the water-dispersible binder, one type or a mixture of two or more of styrene-butadiene copolymer latex, methyl methacrylate-butadiene copolymer latex, acrylic emulsion, vinyl acetate emulsion, etc. can be used.
Among these, water-soluble binders are preferred, and polyvinyl alcohols or starches are more preferred, since they are easy to disintegrate and can be easily recycled as waste paper.

In the present invention, the pigment coating layer contains 0.1 parts by weight or more and 30 parts by weight or less of a binder based on 100 parts by weight of the flat pigment. The pigment coating layer of the present invention has a lower binder content than a general pigment coating layer. As a result, in the pigment coating layer of the present invention, the flat pigments are weakly bonded to each other, and breakage occurs in the thickness direction. The amount of the binder to be blended with respect to 100 parts by weight of the flat pigment is preferably smaller because the bond between the flat pigments becomes weaker, and is preferably 25 parts by weight or less, and more preferably 20 parts by weight or less. However, the strength of the force that binds flat pigments varies depending on the type of binder used. The amount is preferably about 0.5 parts by weight or less, more preferably 0.5 parts by weight or more, even more preferably 1 part by weight or more, and more preferably 7 parts by weight or less, 4 parts by weight. It is more preferable that it is the following. In the case of starch, the amount of starch is preferably about 1 part by weight or more and 20 parts by weight or less, more preferably 2.5 parts by weight or more, and 5 parts by weight or more based on 100 parts by weight of the flat pigment. More preferably, the amount is 16 parts by weight or less, and still more preferably 12 parts by weight or less.

In addition to the flat pigment and the binder, the pigment coating layer can contain additives such as a surfactant, a preservative, an antifoaming agent, and a dye. The pigment coating layer can also include non-flat pigments having an aspect ratio of less than 5. In this case, if the amount of pigments that are not flat is increased, the arrangement of the flat pigments will be disordered, so the proportion of flat pigments to the total pigment is preferably 60% by mass or more, and should be 80% by mass or more. is more preferable, more preferably 90% by mass or more, and most preferably 100% by mass (not including non-flat pigments). Further, the Mohs hardness of the non-flat pigment is preferably 5 or less, more preferably 4 or less, and even more preferably 3 or less.

The coating amount of the pigment coating layer must be 4 g/m2 or ^more in terms of solid content, so that the pigment coating layer has a sufficient thickness and is bonded to the adhesive tape and its end surface (side surface) to prevent destruction. This is preferable because it can serve as a starting point. This coating amount is more preferably 5 g/m ² or more. The upper limit of the coating amount is not particularly limited, but is about 20 g/m ² or less, and even if the amount is increased beyond this, the effect will hardly change, but the raw material cost, drying cost, etc. will increase.

<Backup board>
The backup board of the present invention has a hardness of 75 or more as measured with a type D durometer in accordance with JIS K6253. Since the backup board of the present invention has sufficient hardness and has small bending and denting during processing, it is possible to maintain high processing accuracy and, for example, to reduce the occurrence of burrs due to drilling. In the backup board of the present invention, the hardness is preferably 77 or higher, more preferably 79 or higher.

The thickness of the backup board can be set depending on the type of machine tool, processing dimensions, etc., but for example, when used for drilling printed wiring boards, the thickness of the backup board should be 1.0 mm or more. is preferable, it is more preferable that it is 1.2 mm or more, and it is still more preferable that it is 1.4 mm or more. The thickness and number of paper base materials constituting the backup board can be selected depending on the desired thickness of the backup board.

The density of the backup board is preferably 0.9 g/cm ³ or more, more preferably 0.92 g/cm ³ or more, and even more preferably 0.94 g/cm ³ or more. When the density of the backup board is 0.9 g/cm ³ or more, deformation of the backup board during processing is reduced, and high processing accuracy can be maintained.

Since the backup board of the present invention has an adhesive layer consisting of a paper base material and a water-soluble adhesive, and a pigment coating layer using a water-soluble resin or a water-dispersible resin as a binder, it can be disintegrated in water to form a slurry. The disaggregation process is easy. Therefore, the backup board of the present invention can be recycled as used paper, has a small burden on the environment, and can reduce processing costs.

<Manufacturing method of backup board>
In the backup board of the present invention, the method of bonding paper base materials together using a water-soluble adhesive is not particularly limited, and can be carried out by bonding paper base materials together using a water-soluble adhesive according to a conventional method. Pressing can be performed before or after the adhesive is cured. In this specification, press processing means applying a pressure of 15 kg/cm ² or more, for example, applying a pressure of less than 15 kg/cm ² for the purpose of adhering the paper base material and the water-soluble adhesive. Additions are not included in the pressing process.

When the press treatment is not performed, it is preferable that the pulp constituting the paper base material has a low freeness. Specifically, all of the paper base materials have a pulp freeness (Canadian standard freeness) of 600 ml or less, and at least one paper base has a pulp freeness of 480 ml or less. It is preferable.
The paper base material whose pulp has a freeness of 480 ml or less preferably has a freeness of 440 ml or less, and even more preferably 400 ml or less. Further, in a paper base material whose pulp freeness exceeds 480 ml, the freeness thereof is more preferably 560 ml or less, and even more preferably 520 ml or less. Pulp with low freeness forms stronger hydrogen bonds, so a hard paper base material can be obtained, and a hard backup board can be easily obtained without pressing.
In the backup board of the present invention, it is more preferable that more than half of the two or more paper base materials used have a pulp freeness of 480 ml or less, and all paper base materials have a pulp freeness of 480 ml or less. It is even more preferable that there be.

If the pulp freeness of all paper base materials is not 480 ml or less, the number of paper base materials whose pulp freeness exceeds 480 ml is preferably 2 or less, more preferably 1. .
In particular, when three or more paper base materials are bonded together, it is preferable that the paper base material has a pulp freeness of 480 ml or less in half or more, more preferably 65% or more, and 80% or more. More preferred. In addition, in the case of laminating three or more paper base materials, the ratio of the total basis weight of paper base materials whose pulp freeness is 480 ml or less to the total basis weight of the paper base materials used is: It is preferably 50% by weight or more, more preferably 65% by weight or more, and even more preferably 80% by weight or more.

When performing press treatment, the press pressure is set to 15 kg/cm ² or more as described above. The press pressure is preferably 40 kg/cm ² or more, more preferably 60 kg/cm ² or more. By performing the press treatment, it becomes easy to make the backup board higher density and higher hardness. It is presumed that this is because the pulp is crushed and the water-soluble adhesive permeates into the gaps between the pulps.
When performing press treatment, sufficient hardness can be developed for the backup board regardless of the hardness of the paper base material itself, so the freeness of the pulp of the paper base material only needs to be about 600 ml or less, and 560 ml or less. It is more preferable that it is, and it is even more preferable that it is 520 ml or less. When performing press treatment, the density of the backup board is preferably 0.9 g/cm ³ or more, more preferably 1.0 g/cm ³ or more, and preferably 1.05 g/cm ³ or more. More preferred.
The press treatment is preferably performed while heating. The water-soluble adhesive can be dried by heating during the press treatment, and in the case of a hot-melt water-soluble adhesive, the adhesive can be melted and firmly bonded.

The method of providing a pigment coating layer on at least one side of the backup board is not particularly limited, and includes a blade coater, a bar coater, an air knife coater, a curtain coater, a spray coater, a roll coater, a reverse roll coater, a size press coater, Conventionally known coating methods such as a gate roll coater can be employed.
In terms of coatability, the pigment coating layer is provided on the paper base material in advance and then bonded together with a water-soluble adhesive so that the paper base material with the pigment coating layer is located on the outermost surface. preferred.

EXAMPLES The present invention will be explained below with reference to examples, but the present invention is not limited to the following examples.

- Creation of backup board A base paper with a basis weight of 298.2 g/m ² and a paper thickness of 299 μm, which was made from a raw material with a pulp freeness of 397 ml, was used as a paper base material. This paper base material does not contain waste paper pulp or filler.
"Example 1"
Prepare 5 sheets of paper base material, and add starch to 100 parts of flat pigment (kaolin, manufactured by Imerys, trade name: Capim DG, aspect ratio 11-12, Mohs hardness 1.5-2) on two of them. A water-based coating liquid (solid content 30%) to which 10.0 parts of a water-soluble binder (manufactured by Nippon Shokuhin Kogyo Co., Ltd., product name: MS-4600), which is a type of water-soluble binder, is added, the target coating amount is 10.0 g in solid content. /m ² and air-dried to obtain a pigment coating layer consisting of a water-soluble resin and a pigment.
Next, the two paper base materials each having a pigment coating layer are placed on top of each other with the pigment coating layer positioned on the outermost surface, and then the five paper base materials are stacked together using a water-soluble adhesive (manufactured by Kuraray Co., Ltd., commercial product). Name: Kuraray Poval 28-98 (target solid content weight per adhesive layer: 2.5 g/m ² ) was used to bond them together, and air-drying was performed without pressing to obtain a backup board.

"Example 2"
A backup board was obtained in the same manner as in Example 1, except that the target coating amount of the pigment coating layer was 7 g/m ² in terms of solid content.
"Example 3"
A backup board was obtained in the same manner as in Example 1, except that a water-based coating liquid (solid content 30%) containing 7.5 parts of a water-soluble binder was applied to 100 parts of the flat pigment.
"Example 4"
A backup board was obtained in the same manner as in Example 3, except that the target coating amount of the pigment coating layer was 7 g/m ² in terms of solid content.
"Example 5"
A backup board was obtained in the same manner as in Example 1, except that a water-based coating liquid (solid content 30%) containing 2.5 parts of a water-soluble binder was applied to 100 parts of the flat pigment.
"Example 6"
A backup board was obtained in the same manner as in Example 5, except that the target coating amount of the pigment coating layer was 7 g/m ² in terms of solid content.

"Example 7"
Polyvinyl alcohols (manufactured by Kuraray Co., Ltd., product name: Kuraray Poval 28-98) are used as the water-soluble binder, and 2.0 parts of the water-soluble binder is added to 100 parts of the flat pigment. A backup board was obtained in the same manner as in Example 1, except that 30% of the total amount was applied.
"Example 8"
A backup board was obtained in the same manner as in Example 7, except that the target coating amount of the pigment coating layer was set to 7 g/m ² in terms of solid content.
“Example 9”
A backup board was obtained in the same manner as in Example 7, except that the target coating amount of the pigment coating layer was 5 g/m ² in terms of solid content.
"Example 10"
A backup board was obtained in the same manner as in Example 7, except that a water-based coating liquid (solid content 30%) containing 0.5 parts of a water-soluble binder was applied to 100 parts of the flat pigment.
"Example 11"
A backup board was obtained in the same manner as in Example 10, except that the target coating amount of the pigment coating layer was 7 g/m ² in terms of solid content.

“Comparative Example 1”
A backup board was obtained in the same manner as in Example 1 except that the pigment coating layer was not provided.
“Comparative Example 2”
The target coating amount is a water-based coating liquid (solid content 30%) that does not contain flat pigments and only contains a starch-based water-soluble binder (manufactured by Sanwa Starch Industries Co., Ltd., product name: PLV-500). A backup board was obtained in the same manner as in Example 1, except that the coating was applied so that the coating weight was 10.0 g/m ² .
“Comparative Example 3”
The target coating amount is a water-based coating liquid (solid content 10%) that does not contain flat pigments and only contains a water-soluble binder that is polyvinyl alcohol (manufactured by Kuraray Co., Ltd., product name: Kuraray Poval 28-98). A backup board was obtained in the same manner as in Example 1, except that the coating was applied at a rate of 10.0 g/m ² in minutes.

The obtained backup board was evaluated by the following evaluation method. The results are shown in Table 1.
<Evaluation method>
-Basic weight Measured in accordance with JIS P8124.
-Paper thickness Measured in accordance with JIS P8118.
- Density Determined from basis weight and paper thickness in accordance with JIS P8124 and JIS P8118.

・Hardness After conditioning the produced backup board in an environment of 23°C and 50% RH for more than 24 hours, it was tested in accordance with JIS K6253 using a type D durometer (manufactured by Nishi-Tokyo Seimitsu Co., Ltd., product number: WR-105D). Measurements were performed at 10 arbitrary locations, and the average value was determined.

-Paper Peeling Adhesive tape (manufactured by Nichiban Co., Ltd., industrial cellophane tape No. 430, colored, width 15 mm) was cut out to a length of 3 cm. A total of 8 pieces of adhesive tape were pasted, 4 pieces each in the MD direction and CD direction, so as to wrap around the sides of the backup board. The adhesive strength of the adhesive tape increases over time and reaches saturation in about two weeks, so it was left in a 50% environment at 23°C for two weeks.
After being left for two weeks, each adhesive tape was vigorously peeled off from the main surface, two in the MD direction and two in the CD direction, and the remaining four from the end surface, and evaluated based on the number of tapes that did not peel off. That is, when no sheet of paper peels, the evaluation is 4, which is the best, and when all four sheets peel, the evaluation is 0, which is the worst.

None of the backup boards obtained in the Examples and having the pigment coating layer of the present invention suffered from paper peeling. The backup board after peeling had a step in the pigment coating layer because part of the pigment coating layer adhered to the adhesive tape and was peeled off, but the depth was within the thickness of the pigment coating layer. Yes, it was very shallow.
On the other hand, in the backup board obtained in Comparative Example 1, which did not have a coating layer, the paper peeled off even when it was peeled off from the main surface side and the end surface side. In addition, for the backup boards obtained in Comparative Examples 2 and 3, which have a coating layer containing only a water-soluble binder, paper peeling did not occur when the adhesive tape was peeled off from the main surface side, but paper peeling did not occur when the adhesive tape was peeled off from the end surface side. Paper peeling occurred in both cases. After peeling, the backup board had a large step where the paper had peeled off.

Claims (4)

It has two or more paper base materials, an adhesive layer made of a water-soluble adhesive that adheres the paper base materials, and a pigment coating layer on at least one surface,
The pigment coating layer contains 0.1 parts by weight or more and 30 parts by weight or less of a water-soluble or water-dispersible binder based on 100 parts by weight of a flat pigment having an aspect ratio of 5 or more,
A backup board characterized by having a hardness of 75 or more as measured with a type D durometer in accordance with JIS K6253. The backup board according to claim 1, wherein the coating amount of the pigment coating layer is 4 g/m ² or more. The backup board according to claim 1 or 2, wherein the flat pigment has a Mohs hardness of 5 or less. The backup board according to claim 1 or 2, wherein the water-soluble or water-dispersible binder is polyvinyl alcohol or starch.